Can the retail version of RUSE be entered into Steam?
Regarding R.U.S.E. luckily Ubisoft decided to avoid the bad DRM they implemented in Assassin's Creed 2 and Settlers VII in favor to Steam DRM.
It means that you need to register your retail copy to Steam to play. For this reason of course all physical copies of R.U.S.E. can be entered into Steam.
Thanks to Steamworks features you will have matchmaking, achievements, etc.
It depends. The Steam Store support article about CD keys supported by Steam mentions the following:
R.U.S.E. CD Keys in this format:
Note: R.U.S.E. keys from Ubisoft's Digital River service are not accepted on Steam
If your retail key is in a different format or if you bought the game from the Ubisoft store, well, tough luck.