Well, it would have to be a PS3 hack to play XBox 360 games because

1. The PS3 blue-ray drive could read XBox 360 DVDs, but not vice-versa
2. The PS3 could pre-process the XBox 360 code in SPEs with little impact on the execution stream in the PPE.

Even so, the XBox 360's code runs on 3 PowerPC cores which run at the same speed as the PS3's PPE, so it probably wouldn't work well. On the other hand, XBox 360 would have a hard time keeping up with the massive parallel arithmetic that PS3 games can exploit.