That positioning isn't bad, but I wouldn't expect forum CTR to be as strong as regular content pages - too many repeat views and visits, plus forum regulars are probably better at tuning out distractions like ads.
I must be old fashioned, cause I just place them towards the top (depending on site design) centered, 468x60.
Seems to work.
One of the problems I've seen is top-heavy forum page design, i.e., where the first screen is full of non-content stuff - ad(s), page headers, who's online, instructions or rules, etc. If your ad is in that mess, forum regulars will zip by it without a glance. Putting an ad alongside the posts makes sense if you want to maximize clicks, IMO, although it certainly isn't subtle.
You can also put them right at the bottom of the last message of the thread. A visitor would read the messages, then see targeted ads right there when he/she gets to the end of the last message.
Forum pages can also get a fairly high number of PSA's if the tread is not very active. There often is not enough content (including poorly written subject lines) for Google to to pick up on the key words and deliver an on target ad.
Oh yes. Subject lines can be a problem. I try to change the subject lines whenever they aren't focused enough. At this point I've not gotten a negative response from anyone by doing this.
On my forums I use skyscraper on the right side...then with jscript scroll the div containing adsense down the page as the user reads the posts....so, it is always on screen for them to see.
I use the skyscraper on the left side of the forums. I have other ads there as well.
I agree with the comments above about forums being top heavy so often. Users may just scroll down without seeing the ads at the top. Of course, any ads in forum pages are going to have very low CTR.
I keep mine near the bottom of the page - I'd much rather lose clicks than irritate my regulars.
I've had pretty good results in terms of ad targeting - however, clickthrough rates are low, for reasons mentioned above. I'm not worried though, as it's a hobby site, and even the minimal number of clicks I get will pay for the hosting and some new toys.
I've been quite encouraged by users reactions to the ads - I asked what they thought, and reponses ranged from 'I just ignore them' to 'I like them, they're really useful'. This has me considering moving them to the top, but also giving regulars the option to turn off ads.
One concern I do have is that sometimes users comment on ads in their posts - 'Hey look, we're already getting ads about XXXXXX on here, isn't that clever.' was one. Stuff like that might increase clicks and result in an email from the big G. However, I suspect that my figures are so low it might just get passed off as random fluctuation.