Ok I've moved them to the left again - I think this is the most un intrusive position.
I've seen forums where they have an extremely large banner under the first post; I find this very annoying and think this is an awful lot more intrusive that a 88 x 31 pixel ad that appears off to one side at occasional points in a thread.
To those of you who are using ad blocks - is it really too much to look at something like this
so that I can cover server costs?