Just a thought here.  I am considering a Firewire interface now which will work in my current i7 desktop PC which has a Firewire port on the motherboard.
 
Firewire can live on with a conversion cable will make most of them work with Thunderbolt.  My next laptop and desktop systems will have (available currently in both Mac Book Pro and Asus gaming laptops, and I'll build it into any new desktop that I construct in the next 2-3 years).
 
Here's a strategy that could work:
 
If you buy a USB or Firewire interface to work with the computer(s) you have RIGHT NOW and in the future find that you have need of more inputs, then your second unit (from the same manufacturer) can be a Thunderbolt unit, as long as both units have ADAT...see where I'm going?  
 
Plan A:  If you're still on the computer without thunderbolt, then you'd just connect the thunderbolt unit downstream from your USB or Firewire unit via ADAT, and then the U/F unit into your PC using the appropriate bus.  
 
Plan B:  When you get your new PC or laptop with Thunderbolt, then you'd just reorder the interfaces and connect the Thunderbolt unit to your PC and the USB or Firewire unit downstream from your Thunderbolt unit via ADAT.  
 
Problem solved!  Right?  :)