I want to do exactly the same thing. Don't have an iPod touch yet but am using the Remote Buddy AJAX browser on a laptop to "simulate" an iPhone (until they arrive in France).

So far, it all looks great. I can switch between Airport Express units, control iTunes, select playlists, etc. etc.

I would also recommend buying "Airfoil" from Rogue Amoeba. With that, you can stream anything to your AXs (not just iTunes). I wrote an Applescript that I can launch from within Remote Buddy that:

1) Opens Airfoil 
2) Opens Realplayer 
3) Opens the BBC Radio 1 stream (or whatever stream you want) 
4) Sends it to a selected Airport Express

Can't recommend Remote Buddy highly enough if you want to try and do multi-room music.

Good luck.