Sorry guys, I've been out of the office today with a cold, but I will try to answer these questions quickly (let me know if I missed any):
1) Retail customers play on Steam?
Yes. If you own the retail (v1.02) or Battlefront (v1.00) versions you will be able to download DW on Steam at no extra cost. You obviously need a Steam account and one of the CDs in your drive, but it's not that difficult.
2) Modding on Steam
The files are extracted from the Steam archive with the EXE (and a few other files)
encrypted so that you cannot copy DW from one computer to another and play without paying for it. We control which files must be identical to the archives on Steam (i.e. which files will be overwritten whenever Steam starts up) and these will not include the 3D group, Graphics groups, Doctrines, Database, etc... It will simply be the game EXE and the DLLs most likely.
3) Multiplayer - Steam (v1.03) and Retail (v1.03)
This is precisely why the v1.03 patch is taking so bl00dy long, guys! We want the v1.03 EXE to have all functionality in it for Steam whether you are using Steam or are just a Retail customer waiting for the patch (in which case that Steam functionality will not affect you). With IDENTICAL checksums on the important files, we ensure that ALL customers will be able to play DW together on Gamespy, Hyperlobby, and just normal MP play. This was a very important detail we considered when going with Steam and Valve has been very helpful in implementing it.
4) Price on Steam
The initial price is $36 (10% discount) and you're right it's probably not terribly competitive with the Retail version, but the Steam version will be able for sale for YEARS TO COME while the Retail version will fade from the shelves fairly quickly (as do all retail games). I'm sure the price will adjust over time, so clearly if you would like to wait for that adjustment to happen before buying you are entitled to do so, obviously (you are the consumer after all

).
5) MP Matchmaking on Steam
There isn't any support for this at the moment, but we will investigate it for the next update on Steam. We have been told that it's not terribly difficult to implement if we already support command-line arguments, which we do for Gamespy Arcade and Hyperlobby.
Hope that clears things up, I need a nap.