Hi Guys:

I want to start displaying 300x250 ads to guests only.

When someone logs in, I'd like for the template to change so that the ad is not displayed.

A way of incentivizing registration, and collecting a few extra bones.

Any ideas? I know it's being done elsewhere.

I think it could be a easy implementation in stock code if a custom island insert meets your needs.
Meaning display a ad between the first and second post of a forum.

you can do that now with the ad island between 1st and 2nd post. it is group based

And you can do it elsewhere with some modifications ... I have an ad island on one of my sites that only shows for guests, I also posted a mod "ubb nag screens" at one point that adds a custom island only viewable to guests