According to the "Additional information" section of this page it looks to be a no-no.

Feedback that meets any of the circumstances below is feedback abuse and may be subject to removal:
*bullet point #5 sez: The feedback comment contains links or scripts.

Thanks! But that says "may". Any one have any idea if or how it's enforced?

I would guess it depends on what the person who gets the feedback does. If it's a link to a page that expounds on what a crapweasel they are without their being able to dispute in context they'll probably contact eBay to get it removed. If it's a link to something positive it may not be reported I suppose.
I'm not an eBay fiend, but I've bought and sold casually for a few years and I've NEVER seen a link or url in feedback.
I've sure wanted to have more than 80 characters to say something about a few crapweasels though.