Here's a way of using adsense on a forum and varying the ads position. The different users on a forum tend to use their signature panels in different ways. Some switch them off, but those who choose to display their sig will give you the opportunity to use it to your advantage. Basically, you find the code in your board that displays signatures, then you put your adsense code in there. If sigs are turned on, your adsense will be shown with their sig when they post. Because posters contribute to threads randomly, your ads will be shown in random positions across the threads. Once 3 sets of ads are shown, google won't serve any more for that page. The next page in the thread will be different again as different posters with different sig settings contribute in different places. Basically if users want to plug their website in their sig, they also by default show your adsense ads too. This keeps things fresh, targets well and cuts down ad-blindness. You can fiddle with users' sig settings to tune this further. Blend the ads in to your pages and sit back and watch the results. You may be pleasantly surprised.
Yes, unless your page is done in frames, google will only show 3 normal ads, and 1 adlink. Don't depend on google to do this for you, because if there is anything preventing their filters from only showing that many ads, it becomes your responsibility to make sure you have the correct amount showing.
Be careful, though. I recall something about it not being allowed to make ads look like posts in a forum. While this is in the signature, it's still fairly close.
No one here can, I'd suggest dropping a quick line to google, they are usually very good about getting back to you with a response (save the response, too, to have on hand just in case)
I don't think it's any different to blending your ads to any page, as long as there's an obvious demarcation line between the post and the sig, you're ok. On my forum, the ad is still clearly a Google Ad and not a seperate post. The text colour of the ads is different to the body of the posts and 'Ads by Google' is clearly visible. I've already asked Google if it's ok to have ads displayed this way, and they said it's ok as long as the ad is obviously and strongly seperated from the post, similarly to ones displayed next to pictures. You need to make sure there's some kind of strong demarcation line. Once this is done, you'll be ok, but by all means verify it yourself with Google if you're concerned. I've seen in-thread ads on DP that are blended to look similar to posts, but are obviously still Google Ads.