Somehow on complete accident I ran into this thread and didn't even mean to find this (google sent me here).  Yet I am probably one of the only people out there using the exact setup you're talking about.

First of all, you cannot run this setup on Snow Leopard so don't even waste your time.  You said you know many people doing it but that's simply not true.  Snow Leopard doesn't even support Standard PCMCIA to Expresscard adapters.  They dropped that support with Leopard 10.5 so if you need to run the setup you have, you will be on 10.5 leopard.  I spent many months trying to work with Apple, Dual systems adapter etc and Apple refuses to write support for the adapters (all brands).  Do a basic google search for the adapters.  You will see that ALL of them have a warning right on the box that says "DOES NOT WORK IN SNOW LEOPARD".  Yes, they even put it in bold as I typed it, it's not me being a freak.

With that said, I use the dual systems adapter with my pcmcia cardbus and it works beautifully with driver version 1.73.  Unfortunately RME has stopped writing drivers for this device (unless they plan on being nice and writing an update for people like me who still use this setup ).

However, if you have the "old" cardbus which is the one that is completely flat and uses the "proprietary" RME firewire cable (it's the one that is very brittle and scares you every time you go to plug it in) you don't stand a chance.  I spent 6 months trying to make it work, flashed it, updated it, went through every driver RME offered etc.  It does not work in Leopard or Snow Leopard (but it worked perfectly in Tiger 10.4).

One side warning - if you try to flash your cardbus through the adapter, you're screwed.  If it fails, the card is ruined (you will be paying 150 dollars or more for RME to reprogram the card).  Trust me, been there already.  So do not even THINK about updating that card through the adapter.  If you already have and failed, don't let your pride get in the way.  The card is ruined (unless you have the newest white and green version which I am about to tell you about).

Now if you have the "newER" version of the cardbus (not expresscard) which accepts a standard type of firewire cable and is a green label on the face, then you're in luck.  This is the only one that works in Leopard through the adapter.  Flash it on a PC with a true PCMCIA slot with the newest firmware (which technically is old).  This card also has a "backup" flash on it incase you screw it up. 

If you have the newer version it will work with leopard, I have used both Multiface 1 (which died on me and RME replaced for a Multiface 2 for an upgrade fee on top of the repair fee) and Multiface 2 (my current setup).  I have been using it for 2 years now (The setup is currently running on the machine I am typing to you on).

If you don't meet the requirements above, get the new expresscard version and call it a day.  If you can't afford it, then roll back to Tiger or use a PC or you can go through all the grueling ideas running through your head until you finally give up .  Other then that you will be left with an expensive audio device sitting around collecting dust (Mine sat around for over a year).

Cheers