tedster - 5:00 am on Mar 4, 2011 (gmt 0)
Sounds like they are looking at the word count to ad ratio.
It could be screen usage, too. Google has been using visual page simulation for a while - their "reasonable surfer" model leans on it, and it has modified the way PageRank is calculated.
Last year someone had a penalty reversed because an iframe generated a false positive for their "too much white space" metric. It was documented on Google's own forum, and JohnMu got involved to place a flag on the site, in case it ever triggered that penalty again.
So it could well be more than just word count.