it was developed by Bioware, published by Microsoft Game Studios for the 360, but since then, Bioware was bought by EA, which is pretty much a multiplatform company. Demiurge Studios ended up publishing the PC version. Im not betting on the series coming to the PS3, but its not impossible. I wont be too surprised if it eventually did
Yes, the PC version was published by EA, but on the Games of Windows brand, which means that, it still is on a Microsoft platform.
It´s not impossible to go to the PS3, but Microsoft will not let it go without a fight, Mass Effect is huge, not only a huge IP, but a huge universe, and if there is one thing that Microsoft is great in doing is use all potential of their universes (something that Sony is not, so far), and the best example of that is Halo.
If you look at Halo, it´s an FPS about a character named Master Chief, but if you look closely, Halo is set on a war, which means that an a prequel RTS (Halo Wars) or a squad-based "Call of Duty 4 meets Gears of War meets Ghost Recon" without Master Chief (the rumored bungie project) can be done without feeling "forced".
I expect the same "universe" with Gears of War...and the only PS3 game that can make something like that is, imo, Resistance.
Buttom line, I don´t think Microsoft will let it go, and, with all the buyings (or tryings) going around by EA, i don´t think that can keep totally away from making 1 or 2 exclusives for whatever console.