From joystiq.com:

Craig Alexander, Turbine's vp of product development, spoke at Austin GDC today in a panel called "MMOs to Consoles". He did spend a lot of time pointing out how difficult it is to develop games on the PS3 vs. the Xbox 360.

Here's the logic:
* The Blu-ray drive on the PS3 is actually slower in seek and access time than the DVD drive on the Xbox 360. Once the Blu-ray gets up to speed, it can handle large amount of data. Just not in seeking and accessing, which is a pain for streaming when you're playing something like an MMO.
* The memory architecture is different. When you're dealing with main memory, you work with "256ish MB" on the PS3 vs "512ish MB" on the 360.
* The tools aren't as friendly on the PS3. That includes the performance monitor tools, the debugger tools ... everything.
* Migrating a project from the Xbox 360 to the PS3 is much harder than moving from the PS3 to the 360.