Uhhhhh no.
If UT3 was any indication for the PS3 for what console Steam is going to be like, then maps would add maps, but I heard no console Steam What-so-ever for this game.
Steam allows everything and supports mods. You have access to all your files, even hidden one (Through 3rd party programs). Are you getting confused with GFW-Live? GFW-Live is a great idea but executed terribly, like if they actually had people working on it, this idea of connectivity between Xbox and PC would be fantastic. Despite that, even GFW-Live allows for mods and maps if the game was designed that way.
Edit: I understand that you are saying that now they developers the option to do things similar to DLC instead of giving maps for free, but its all about who makes the game. Steam allows it but it doesn't enforce it like Microsoft does. It's all about the developer. If they charge for maps, they are probably too greedy to allow custom maps, too. But "God No!" Steam does not prevent access to player made maps.
Steam has no control over prices. [censored] MW2 is still expensive and is 65 dollars. It all relies on the publisher and not where its sold. All foreign prices are insanely expensive everywhere and not just on Steam. The only case of Valve doing anything wrong is that some of their prices in the TF2 store is rounded out, so pound-euro-dollar things are somewhat off.
I actually meant Deathball for UT2.
As for Steam allowing for mods and custom maps, yes I know it can, but more to the point, it allows the developers to lock modders and mappers out, so that the developers can extend their own profits with map packs, etc, but limit the creativity of the community.
I know Steam doesn't control the prices, but I still object to any digital platform telling me that I have to pay more than any other country for exactly the same thing (sometimes for less with our crappy ratings system). Call it irrational if you will.
So are you saying Bethesda is going to allow modders and mappers? I doubt it. Not if they can make another $20 mill profit from releasing a map pack in a few months time.... and there you have it. It all boils down to money in the end, doesn't it? (and that's exactly why Steam won't let me sell or gift my old games, too. They'd rather be making more money by someone buying it from them.)