I've noticed that one local restaurant site I run is doing great and another has vanished. They are reasonably similar in terms of structure but have totally unique content so no dupilicate issues.
Both sites have DMOZ listings and a few other good links - PR is similar (higher on the site that has vanished if anything)
Both have .co.uk domain names
The biggest difference is the server they are hosted on. One is hosted on a UK based reseller account and the other a US based one. The one on the US hosting is fine, the one on the UK hosting is gone. Besides IP address there is a small amount of cross-linking between some of the sites on the UK server - as a local webmaster I imagine this is pretty common.
Anyway that's my most controlled observation on this update.