There has been a lot of talk about GPL being incompatible with Microsoft market place (as with Apple’s appstore). For example this Arstechnica post from 2011. http://arstechnica.com/information-technology/2011/02/windows-phone-marketplace-bans-the-gpl-and-the-app-store-should-too/
I think that this is pretty important topic now with WP8, since we could use so much GPLv2 licensed C++ libraries with our app, that it really hurts to think that the GPL is not allowed in the marketplace. However, that is not true anymore! I just downloaded WordPress client from the marketplace, and you had to accept GPLv2 license before using the software. It seems that GPLv2 is acceptable now! However, I’m still pretty sure that GPLv3 is not compatible with appstore, since it has some DRM system on it.