A while back Simon asked me to explain my preference for MIT license (as opposed to GPL) for my projects and keep it civil.
Despite the hardship of latter condition, I managed to put together illustration of how popular software licenses play with each other (or don't).
Compatibility of licenses
I prefer MIT because it has affinity for code to drift around, without strings attached. GPL doesn't play well with other licenses, including the different versions of itself.
I have projects which I can't release sources of, because of GPL compatibility issues. Sad panda.