Will it ship with an external PCI Express card or will it be available from rme?

Sonnet also has a Thunderbolt to External PCIe system: Sonnet QIO Thunderbolt Interface Kit

Edit 11/29/2013: Unfortunately the above adapter does not work. It is fixed to PCIe 2.0 with 5 GB/s, while the MADIface XT uses PCIe 1.1 at 2.5 GB/s. That is only a problem when the used interface is not downward-compatible, which seems to be the case here.

Therefore at this time we can only recommend the Matrox TB to PCIe adapter as direct solution.

I would say a strong requirement on reliability is what inspires the choice of locking connectors on both sides of this adapter cable (and the interface box, left-side at least).
On external side of the adapter cable you see a D-Something (screws on both sides), while onboard the interface box there's an SFF-8088 connector that locks in place without having to turn screws.

And, reliability in connection aside (these can't be pulled off inadvertently, something you can't say of FireWire, USB or Thunderbolt connectors), there's bandwidth issues to dictate what you can use, and what not. The SFF-8088, for instance, is a common feature on high-performance, enterprise-level storage solution and DSP/Control buses (also called a mini-SAS connector, Serial Attached SCSI), which shall grant a throughput of about 10 Gbit/s if memory serves me right. Which means video, real-time processing, heavy and sustained traffic indeed.

Consider, in addition to all this, that a manufacturer has to opt for a connection solution that not only copes with the technical specifications of both the device and its range of applications, but a connection solution that can be sourced from a possibly unlimited number of vendors because you might be forced to review your production costs, and/or because you might be forced to re-locate production elsewhere,  and because you want third parties to develop devices that connects to the one one you're designing.

Another factor that's worth considering, too, is the time with which a manufacturer developing his own products must market a new product after it's been designed and built, which strongly suggests that you shouldn't reinvent the wheel when designing your products, as this would greatly slow the final stages of the design process (you need to source from suppliers, not get bobby-trapped into a blind alley with a connector that only you can build, and in the hope that it will work flawlessly for the next 50 years.

I haven't got the prescription glasses on my nose as of now, but at first sight I'd say the D-Something connector is of the type widely used in high performance computing, which would justify serious business at expansion cards level anyway.

I have contributed a few links on the other thread, and thought I would chime in here with a big thumbs up for the PCIe x1 connection method. The PCIe interface protocol is the work of the PCI Special Interest Group or (PCI-SIG), who manage and determine all of the PCI and PCIe specs and requirements, both in terms of hardware, software, signalling etc... Basically all of the technology that goes into making PCIe what it is, as found on every motherboard you purchase, as well as it's external variants. The PCI-SIG is comprised of a massive consortium of over 900 companies, in contrast to the limited and more proprietary licensed nature of Thunderbolt at the moment. As Matthias has mentioned, external PCIe is used in higher end video set-ups, hard-drive arrays, expansion chassis products and larger industrial setups. External PCIe is basically a really fast simple interface, that extends PCIe speeds outside of a computer chassis. It doesn't have the nifty daisy-chain approach of Thunderbolt, because all it is set up to do is provide a stable high bandwidth connection to an external device. Importantly it also provides a locking connector, the lack of which can often be a problem in high bandwidth professional environments. (see here: http://www.gearslutz.com/board/moan-zon … ctor.html)

Companies using this external PCIe technology include Magma, Sonnet, Matrox:

http://www.magma.com/pcie-x1-host-card-eex1
http://www.magma.com/expresscard34-host … aptop-ex34
http://www.sonnettech.com/product/qiopcie.html
http://www.sonnettech.com/product/qioexpresscard34.html
http://www.sonnettech.com/product/qiothunderbolt.html
http://www.adorama.com/1/1/31970-matrox … e-adp.html
http://www.adorama.com/VDMREXP34ADP.html
http://www.adorama.com/1/1/161587-matro … t-adp.html
http://www.ioi.com.tw/products/prodcat_ … ostID=2053
http://www.ioi.com.tw/products/prodcat_ … ostID=2053

Here's some general external PCIe info:
http://www.pcisig.com/specifications/pc … ations#ecn
http://www.pcisig.com/specifications/pc … abling1.0/
http://www.extremetech.com/computing/87 … lternative

Madiface XT videos:
http://www.youtube.com/watch?v=ijPLMZJTVP0
http://www.youtube.com/watch?v=7qh63FNQslA

Also, please remember that PCIe x1 is only one of the two choices available on the Madiface XT box, the other being the ubiquitous USB, which offers a simple connection method without the need for any additional cards (on both Desktop or Laptop). USB3 unlocks all of the units i/o, whilst USB2 simply reduces the channel count. Given the stability and success of RME's other USB products, I think you can be assured of a robust solution.

The Madiface XT ticks all the boxes for anyone using Madi, with the useful addition of a few analogue i/o and as well as the expanded functionality of TotalMixFX compared to the standard TotalMix. The lack of any analog i/o was a major complaint from users of the Madiface cards, and with fewer laptops offering ExpressCard slots, this product seems like a great solution that can be used both in a permanent studio as well as on the move. For fans of Thunderbolt, Sonnet and Matrox already have thunderbolt to external PCIe x1 adapters available, or you are free to use USB. I fail to see how this abundance of choice constitutes a problem for the end-user? If anything, it offers a variety of flexible connection methods that can be tailored to your individual needs or to work around the problems of a lack of a specific interconnection type on any particular workstation. A win-win I would have thought.