I know of 2 forums in the past week who have received a compliance emails from Adsense and had Adsense disabled stating it was in violation, and both boards were using the built in $ad_location[ad_header_end] function to display Adsense. I think Google have lost the plot, because neither were using any sort of "floating box" or iFrame and both were very standard sites. So if you are displaying Adsense on vBulletin, you may want to get your site reviewed for compliance by Google because it seems their detection is "broken".
Thats the thing, the sites didnt have either an iFrame or floating box and the ads and display code was no different to the millions of other boards. After all it was done with the Adsense location built in to vBulletin just for that purpose. This means either vBulletin has a major problem or Google is screwing up banning sites for nothing. The code was sound so it looks like the latter.
I will says using adsense in forums is not easy , You must have mail them and ask about this if you have not used such boxes .