Robert_Charlton - 7:40 pm on Jul 1, 2010 (gmt 0)
In my experience, changing a site from bloated code to good CSS shouldn't result in even a hiccup... as long as everything else is the same.
I've seen sites, though, where developers... really good developers... have done things like leave a meta robots noindex tag on the development template after the site's gone live, thus completely blocking Googlebot. Mistakes do happen.
Can Google spider your site, and are you getting indexed? Have you done a search for an exact text string? Etc etc. At this point, it may be worth rechecking some very basic stuff.
Another option, of course, could be that Google has goofed up. To speculate about possibilities... maybe Google was getting gamed via the new instant index feature of Caffeine and implemented some sort of quick fix that's kept you out too long. I can't believe, though, that recoding with no content, filename, or navigation changes would be something that they'd want to discourage.