I run a forum that is open discussion and the threads therefore run the gamut from TV to politics to travel and everything in between. The ad units being served are almost always about parenting and kids while the threads themselves are so varied. So on a thread about video gaming consoles you'd expect related ads but they are about fixing your marriage. My question is does G sort of default to an overall "theme" for forums instead of match threads? Should I edit templates to make G-bot put a focus on the thread content when deciding what to serve? Anyone have an experience like this?

I have noticed this as well on other forums. I think it may have to do with the fact that there is so much text on a forum page that is irrelevant to the content such as, user names, forums signatures, location information, etc. It may be difficult for G to know exactly what the page is about. Until we gain access to the algorithm they use to determine this information, it will be impossible to tell.

However, if your adsense ads aren't performing like you think they should then look around for alternatives, there may be something better for you out there.

I own a forum that has Adsense ads displayed. It is quite common that the ads are not related to the topic of the thread. I think this is a common problem to msot forums.

I have just implemented section targeting and I'll see how it goes for a bit then maybe if things don't improve too much I will also use the google_ad_section_start(weight=ignore) directive around all other page content. I'll report back my findings.

Yeah, it's best if you try section targetting. That's what I had to do with some pages of one of my sites and that worked. Don't know if it will work for you though, but if you don't try then you will not know.

So far so good. Using targeting I have found that many of the threads now seem to have relevant advertising. This had only been a problem on the open discussion forum, whereas other forums I own which are topic-specific do not (or have not) need(ed) ad targeting.

This is very unfortunate. It happens on my forum as well.
I do remember hearing of, but sadly forgetting exactly what, but there are tags you can use to place keywords in for AdSense targeting. It'll only display relavent ads pertaining to the keywords you placed in the tags...wish I remembered what the tags were.