This is actually a rather severe misconception regarding the GPL. Someone using your sources DOES NOT have to make their application open source! They only have to allow access to modification to YOUR sources they may make.
The GPL is NOT a viral license like people may try to claim.
And furthermore, as a side note, 'making sources available' does not necessarily mean posting them up publicly for just anyone to grab. The GPL does not negate *selling* a derivative work and only allowing paying customers to have access to source codes. They do still however have to make their modifications to your source public, if your sources were public in the first place.
I think it's hilarious that we're splitting hairs on licensing issues between homebrew devs but blithely trample the rights and IP of Sony with every offical-sdk based release. I'm not saying we should or shouldn't, just be aware of the incredible irony there