While they give the engine a new name (IW 2.0, 3.0 and 4.0) to my knowledge (and in my opinion), the capabilities of the engine are still somewhat limited. From what I've seen, the Source engine, while old, is far more versatile than the IW engine.
FYI here's some of the upgrades the Source engine got
http://en.wikipedia.org/wiki/Source_...table_upgrades
Sure it's not as often, but the Source engine was SPECTACULAR when it first came out. The physics that exist in that game are still very, very good.
redit:
When have they ever release a game in a timely fashion. They're just as bad, if not worse, than Blizz :P