Valve understands their user base well enough that they made the source engine scalable from direct x 6 class hardware all the way up to the latest dx 9 shader technology, which is no small task. They're clearly trying to make their games as accessible as possible. I have to believe they understand that a large percentage of potential customers don't have a broadband connection, or any internet connection for that matter. I can't see them making decisions that run counter to the major effort in engine scalability, by making the game less accessible. It's possible, but doesn't seem likely to me.
I think steam is an interesting idea. Valve is trying to do something most of us have been hoping for: game developers taking a bit of control back from publishers. It also allows them to combat cheating more directly and make patching easy (from a mass marketing standpoint.) More computer savvy users may not like these latter bits, and how valve handles this will likely play heavily in steam's future. Of course, right now it seems like everyone is nervous and looking for the devil in disguise, probably because valve is the first large game company with a large customer base in place to try something like this.
It'll be interesting to see how it evolves.
just my $.015