I suspect it's going to be a lousy time for ads in the near term.  Less money spent on them; less readers clicking on them (on the idea everyone is spending less).

I saw a decent thread on various ad systems over at webcomics.com -- I'm really curious about how January shakes out for creators running ads.  I'm currently using Joey Manley's new advertising platform - Webcomics World - but it's too soon to provide any feedback on it yet.  Previously I used a combination of Blog Ads, Project Wonderful and Google Ads.  Google ads have never worked for me, PW has always been a small amount (but consistent) and Blog Ads used to be pretty good but declined severely over 2008.

NOTE: I grabbed this link from the webcomics.com thread - Tony Piro wrote up an overview of many ad networks - a little out of date but still, a pretty useful overview.  One other new network you might also investigate is from Six Apart (they own typepad and moveable type) - I have not used it but did consider it last year.