a) Developing multi-threaded games is very difficult because of problems with predictive branching (you can't effectively split some tasks among multiple SPEs because the results of one piece may invalidate work being done on the other SPEs)
b) There is a little less flexibility on the memory architecture of the PS3. The PS3 has 256 MB of system and 256 MB of video memory, while the 360 shares 512 MB between the two. While both systems have a combined 512 MB of system and graphics memory, if the 360 doesn't need as much memory for the system, the rest can be used for graphic textures
c) The 360 employs some texture compression that allows for bigger textures with the same amount of memory space and a bump in throughput, as well.
I believe there a few other areas that supposedly give the 360 some advantages, but those are the three biggies - (plus the supposedly better/easier SDK).
That said, I've seen some really impressive games for the PS3 which makes me wonder the validity of the above.
From what I've read, it's my understanding that: