I've heard these problems back then; and the reason I'm curious about this again, is that my pr 5 site suddenly got delisted today but when I search on my domain name it still exists on the first line of google, so most likely not penalized but for most keywords(good volume above 20k monthly search volume keywords) that my site use to rank on the first page of G is now completely gone.
It's also a site that is more than 2 years old but less than 3. So I don't think it's brand new site sandboxing.... which I'm kinda aware of.
So... 1. possibly not penalized... 2. possibly not sandboxed... 3. can be proxy or 302 redirect hijacked? - I also search on a long string that only exists on my site with the quotes(ie. "long string"), it only shows my domain no other domains/proxies. - I just added the <base> tag - I also added reverse dnslookup and verify googlebot, msnbot and slurp and dump meta noindex or index if they are the right bots. 4. link building was a little stronger the past week since it got picked up by several different sites.