I really like that second thin black one I have to say. Only thing I'd maybe say was the possibility of varying the tag line. You could have 'The best XM site on the net', or something like 'XM tech advice, parts, chat, consuelling (ok maybe not that bit!). The only thing with 'long' words like 'enthusiasts' in a small font like that is it may fail to be read in a passing glance, but then again as you said Andrew, chances are a fellow XM owner would be checking out your car anyway.
Other than those, the simple wide, thin URL ones (about the size of the latest two you've produced) work very well, and are quite subtle, i.e the kind of thing you wouldn't mind putting at the top of the windscreen or rear window. I do think the 'split' sticker idea is a good thing to reduce costs too.
Ciarán