As I said before, the way I understand it is you may create a derivitive work, and even charge for it. However, because of the GPL all source code must be released, so making a free version will never be impossible.

And yes, you can get the free source code from www.limewire.org. Of course they don't distribute a free (and ad-free) compiled program on limewire.com, they wouldn't make any profit that way. But you can search for clean Limewire, which if I remember correctly is hosted on a geocities site somewhere (though it gets subjected to heavy loads).