In an interview, Newell said they'd have it to Vivendi by August. (Provided no more hackers come along). VALVe is probably finishing up things, doing box artwork, those little pre-release things. I doubt they would re-vamp the graphics engine, that would take way too long and be pointless for a game that has come this far.
After HL2, I suppose VALVe will work on Steam, as from what I can tell, they plan to license it to businesses for their programs, and they'll probably develop patches and levels and such for HL2 and CS2. They might possibly work with Gearbox to develop (hopefully) OpFor2 and BlueShift2, and after that, probably start work on a new game/game engine, or some development tools.