@Brian:

I think there is a significant, critical difference between "more permissions" and "more restrictions".

In fact, the CC license generator describes the "More permissions URL" as:

A URL where a user can obtain information about clearing rights that are not pre-cleared by your CC license.

So, I don't think the license intends for such a link to be used for adding restrictions, but rather for indicating how to add further permissions/rights.

As for Brian Gardner's approach: I think it's just fine - at least, with respect to adherence to GPL. I'm curious to see how it pans out, long term, as a viable business model, though. If it indeed works, then I'm happy for him!