I run a 300x250 ad inserted between the first & second post that is shown to guests only. I tried putting an ad within the first post a while ago but hated how it looked so removed it. If I recall correctly it didn't perform any better than the ad between 1 & 2.
Although it may annoy some, it's a worthwhile experiment. Personally I wouldn't show the ad to logged in members though. AdSense optimization is all about experimenting. Find out if the ad performs well, and see how members react.