Update again: it looks like you're hiring professionals to do this. What a shame. There is so much talent here and the logos for the current SE sites were all designed by users.
Update: With some of the proposals getting close to the beta stage, I really think that this needs to be answered. I'm eager to fire up Inkscape and get sketching!
When a proposal meets the threshold for beta, where do you get the logo for it?
Could we have a mini-contest for each site? The prize could be a certain number of points on that site or even just an acknowledgement.