Fair enough, LD; I wasn't aware Microsoft let you do cross-platform multiplayer in Live stuff (they don't exactly promote it, or actually include it in anything nowadays).
There was also Final Fantasy XI on the PS2, and a handful of PS3 MMOs, so in that regard cross-platform multi isn't a big deal. What is a big deal is that this is actually Steam on PS3, with all the caveats of such, including updates that completely ignore any of Sony's procedures. I would guess it also means Source features like dedicated servers.