This is a nice idea, but the sponsors have already paid for advertising and you want them to kick in more for a specific group?
For now we'll just go the way we are, if people are happy with the site and what we're trying to do, they can contribute $$ to keep us going, if not, that's fine, and there is nothing special done for supporters currently.
Based on past experience, I know that making contributors 'special' is a big no-no to some people and just introduces problems. We may try to do t-shirts or a sticker pack or something, but nothing has been decided yet.
Also, based on our server load this week and the amount of attachments we have, we'll have to move to a dedicated server in a few months. That will almost double our monthly costs.
Currently we have around $1200 in the paypal account and with $109/month hosting fees (plus domain registrations coming up) that's about 11 months' worth. The dedicated server will be ~$200/month plus a setup fee so that money will deplete pretty quickly once we have to move that way, but it will allow us to double our bandwidth and have 12 times as many attachments.