Post #51 has all the info I could find so far about the windows release. Steam may come later, seems unlikely it will be on Steam when it launches going by what they said. I imagine they are simply incorporating their own system for the DLC initially, so they don't have to pay Steam or someone else the 30% cut. Though that does seem trivial when they're already paying it to Sony/MS/Apple/etc, so perhaps not? They did say pricing of the tables would be higher on certain platforms where the 99 cent / lower prices won't fly. Probably referring to the console versions.
Did they ever say how this was coming to windows? I'm still leaning towards the xbox360 version since my wife seemed annoyed when she realized I was thinking of getting the PC version.
Still though, depending how it's released I may consider it.
They do plan on looking in to supporting some pretty neat stuff on the PC, virtual pinball machines with 3 displays and motors, and I'm sure if there's any sort of community for mods/custom builds, it'll be on PC. I don't know how likely that is though, given they generally don't want people changing stuff under the hood iirc.