Here is a portion of the interview from Computerandvideogames.com:

CVG: You've already expressed your plans for Mass Effect to be a trilogy. Do you see all three games being released on Xbox 360?

Zeschuk: Certainly out intent is to have all three iterations or instalments of Mass Effect to be on Xbox 360. That's the goal.

CVG: Exclusively to 360 all the way?

Zeschuk: Can't really comment on that aspect yet, it's too far into the future. Thinking a year in to the future is a big challenge these days.

The law we can't control is the length of the console cycle. I hope, I hope, I hope that this cycle is longer than the last one. We kind of just got started last cycle and it kind of ended when our second game came out. Downloadable content could also bridge the gap by giving you new little islands of story between the big game releases.

Source: CVG