Microsoft put out a press release saying that ME1 is an exclusively 360 experience. "If you want to the full Mass Effect experience (minus the apparently bad iPhone game) you'll only find that on the Xbox 360" or some such.
Don't forget ME1 is also available for PC.
But I agree, ME1 should be released for PS3 as well. A special Trilogy Collector's Pack would be awesome once part 3 is released.