Microsoft Smartphone Code Signing and the GPL?

spacemonkey asks: "I am a professional developer, but in my spare time I have been developing games for the Microsoft Smartphone platform. Included in this work is a port of gnuboy a GPL gameboy colour emulator. Where does the GPL stand on the question of codesigning applications where required? Basically gnuboy is available, with full source for smartphone, however there are a large number of users out there who are unable/unwilling to remove the certification requirements from their smartphone devices, so to allow for these users, I need to sign the code. To enter into the code signing program will cost me approximately £500. I am interested in signing the application to make it available to a wider audience, however since I am not running a charity I was wondering whether charging some nominal fee for the code signed version was compatible with the GPL or not. So users would have an option on a signed version for less than £5, or an unsigned version free, which will include the full source code. Am I allowed to charge for GPL software in this way, where the charge is to cover the packaging of the application into a signed form?"