Just to echo the other people on this thread who say that they see PageRank, I checked and it looks like you have plenty of PageRank internally as well. I didn't see any manual webspam actions that we'd taken on the site either. Have you actually checked in the Google Toolbar? That's the only official place to see our toolbar PageRank--lots of other sites and plugins can be broken in various ways.

In general, PageRank tends to drop for one of two reasons. The first is url canonicalization issues (e.g. messing up http vs. https, www vs. non-www, rel=canonical pointing to weird places like 404 pages). The second reason is selling links that pass PageRank, which violates our guidelines.