I actually did get the ExpressCard to Cardbus adapter to work with my old Toshiba Laptop PC with Cubase 3SX and the RME HDSP Cardbus card. I had a weird IRQ BIOS error message on start up but once I bypassed the message and continued into Vista the RME driver was recognized and I recorded audio with no issues. However, this toshiba was not very powerful machine so I was just using it to record my guitar with just Guitar Rig 3 and nothing else going. It worked fine. But this was not a high powered setup or robust system to say the least. I wasn't pushing tons of tracks with tons of VSTi plugins, etc.
The ExpressCard adapter was from a company called Addonics.
Flash forward to today and I wanted to get a more powerful laptop system so that I could really use the Multiface to full advantage. So I bought a new MBP 17" with the ExpressCard slot. On a lark, I tried the same adapter and HDSP card and wouldn't you know, Snow Leopard did recognize both the adapter and the RME card but it stated that no driver for the RME card was installed so of course I can not use it. After checking the RME downloads it looks like to use the snow leopard driver the firmware version has to be version 18. But this version is only for the HDSPe card (ExpressCard Version) and not the HDSP version. I'm sure this is because Apple does not have any machines with PCMCIA slots for a long time now.