Any reason you don't want to run the latest Xcode that Apple provides for free in the App Store?
Probably not so much these days - used to be that some things didn't support Xcode 4. Also you need Xcode 3 to build for old OS X versions.
Now, we may be getting to the point where everyone starts moving to Xcode 4 and dropping OS X 10.5 support - that's presumably the reason behind Gavin's question. But, the fact that Gavin is asking this question means, I think, that like many open source projects Bitcoin is currently still building with Xcode 3. That's as good a reason for me as any to stay with 10.6 so I can still use Xcode 3. (Xcode 3 won't run on 10.7.)
You can always setup dual boot (Mountain Lion & Snow Leopard) if you want to run older software.
Because repartitioning my disk would be a pain. And it's a laptop so adding an additional disk isn't really an option.
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