In GWT when it shows you 404 errors, it will show you what pages they were referenced from. Recently I have found them showing 404s referenced from 2008 sitemaps, 2010 sitemaps, even from other sites, etc. as if every post or page ever created and referenced in a sitemap is supposed to exist forever in that same location. I realize that is the point of "permalinks" but the reality is that old stuff gets removed. I see the same kinds of 404's on static sites so I don't think it is WP specific.
I am seeing more weird 404's than ever, sometimes the URL is obviously search results, I have quit pulling my hair out over GWT errors that make no sense.