mhansen - 7:47 pm on Feb 14, 2012 (gmt 0)
We run adsense on one of our busier sites... We also sell ads direct. It's not at all uncommon to visit a page of our site and see the SAME advertiser who is buying the ad space directly from us, also showing in the single adsense block at the very bottom of the content.
Who is to say which came first? What if you decide to sell ad space to someone this week, and in 3 weeks, they also run an adwords campaign? Then what?
I think you are worrying about a non-issue here. All said however, I agree completely with what everyone else says about burning the bridge. If Adsense is your primary source of income on the site, it may not be worth your short term gain to mix up the kool-aid. Adsense probably has a pretty good idea of the quality of your site visitors as they relate to advertisers, and the last thing you want to do is sell a direct ads, and have the perceived quality reduced.