I just took the clear bra off the Crossfire that was professionally installed 3 months ago. It had yellowed, but it wasn't really evident when it was on the car. My XFIRE is SSB, so thats probably the reason I couldn't see the yellowing. We are a bit closer to the sun here in Denver...maybe more UV light...but it seems to me that this happened pretty quickly.
Anyways, I'd rather go the chip repair route than the vinyl bra route at this point.