For some reason they don't appear in some locations, like in here. Perhaps the "Support" keyword triggers something that tells their engine not to deliver certain ads.
Final update on these for a while I reckon: Performance was a little down since I replaced the wee text unit above with a large one for guests, so I've put it back the way it was. The text unit is less obtrusive and I'd prefer it that way anyway. Bizarrely, in both cases the clicks - AdSense reward for clicks, not views - still came mostly from the 468x60 at top-right; I can only guess that the other ads drew attention to the standard one or something.
I've moved the big ad at the bottom of the page up slightly towards the content on the forum home page, the forum listing page and the thread view page; I think it provides a useful service while not being too obtrusive. I've also added a guest-only large ad that appears after the first post in each thread; registered members won't see this. That'll be it for a while, I very much doubt I'll be adding any more ads, as I don't want it to be in people's faces. I may move them around a little though.
I've never mentioned btw: Thanks to the people that do click on the ads. It provides a very useful revenue stream that allows me to spend more time on the site, and I'm hoping will allow me to eventually move the site to it's own server, possibly even in Ireland. I think they provide pretty good keyword matching - I wouldn't host them if I didn't think they did - particularly at the thread level, so I hope you guys are getting something out of it too.
adam