So post a click-through banner ad page for those not logged in, and hell, I wouldn't mind a banner where you've got the text ads now. Why not go with the Google text ads instead of what you've got going on, at least some of the time?
I like this idea -- how hard is this to do? Alternately, I know that a lot of sites have banner ads on their free version and also offer an ad-free version for paid subscribers. I'd gladly shell out for an ad-free version if it means easing the burden on poor old Jough.
I personally think that banner ads on the main page would junk it up -- I like the sleek, minimalist look it's got now. I'd prefer a pop-under (I've got MoZo, but I've already opted to allow pops on this site on account of the PM feature) or a clickthrough page, just to keep the crap "outside the gates," so to speak.
Furthermore, I think Jough's right about the tip jar not being the most equitable way to collect. There's a reason we periodically delete the accounts of serial lurkers: we don't like them. They use more bandwidth as a group than the regulars, because there are just plain more of them -- I say we make them carry their load.
And since, by definition, lurkers don't post, who's going to complain?