This is something that I find puzzling about the current generation of game consoles. In the past, one of the pros about console systems vs PC was that people usually didn't need to worry about framerates. They understood that while the visuals might not be as good as a PC title, at least they didn't have to worry about uneven framerates due to system requirements. Most console games always had a more or less consistent framerate. It seems that with the PS3 and Xbox 360 however, quite a few titles suffer from framerate issues.
For the most part they run smoothly, but there are occasional problems with slowdown. Mass Effect had them, for instance, even though the 360 was the primary platform. Two Worlds, a PC port, was the worst example I've seen. Every few minutes, it would come to an absolute standstill for 3 to 5 seconds. I'm not exaggerating. It was rather distracting. Fortunately, framerate issues aren't something that bothers me unless they are really bad, like in TW.