The latest HalfLife2.net forum thread has info from Valve's Gabe Newell on the three different versions of Half-Life 2 they plan to release. Apparently the plan is to release a single-player only mass market version ("sold mainly at the Costcos and Walmarts of the world"), a traditional single/multiplayer version for places like EB Games, and a collector edition's version ("with lots of cool bonus stuff for people who like cool bonus stuff"). In addition, Gabe reveals Steam pricing plans

In the Steam world, some people will want to buy it once, like the middle SKU above. Other people will want to buy the game on subscription (e.g. $9.95/month). The good news for the "buy it once" crowd is, well, they only have to pay once. The bad news is that when we come out with new content (expansion products, TF 2, and presumably other games) then they have to pay separately for those. We're pretty sure that the $9.95 guys are going to get the better value, as we've been pretty good over the years at generating a lot of content. ...

update: Since it seems people need it made a little more clear, here you go a nice translation from our local informative whore timm:

1: Retail box, singleplayer, no multiplayer - ONE TIME FEE!
2: Retail box, singleplayer and multiplayer - ONE TIME FEE!
3: Retail box, singleplayer, multiplayer and other unknown cool stuff (collectors edition) - ONE TIME FEE!
4: STEAM, singleplayer and multiplayer - ONE TIME FEE!
5: STEAM, subscription model. Pay a montly fee and have access to all Valve titles, including the stuff on STEAM now (HL, OpFor, CS, TFC, etc), Half-Life 2, all future stuff they put out, and so forth.

Second, patches (bugfixes, maintenance releases, etc) will be free no matter which of the six option you decide to go for.

Community-based mods will be free unless the authors want to charge for them. Free community based mods will/can be made available though STEAM free of charge.