After playing Portal 2 on PS3, the multiplatform experience was a breath of fresh air. The PSN is not a money-maker for Sony -- just a reason to buy their console.
Steam already works on PS3 -- no more porting needed. Merging two large communities of players gives you a huge competitive advantage in the market over Microsoft and studios could publish truly cross-platform titles (competitive FPSes? Probably not. RPGs, strategy, racing, rhythm, fighting games? Sign me up!)
Lastly, Steam already owns the online PC market -- with the notable exception of Blizzard -- and they have something to gain here too. In the 10 years of its existence, I have never seen Steam down for more than a few hours and it'd give a chance for Sony to rebrand its online experience to something that's more reliable than the XBox live and still free.
Will Sony do it? Snowball's chance in hell. Would it be revolutionary to have a console/PC agnostic matchmaking/gameplay system? Yes. Do gamers want it? Every review I have seen about Steam on PS3 has been glowing or at the very least, having no complaints. The engineering's been done. All Sony would have to do is show some leadership.