I just went into the Google webmaster tools and half my sites which were all previously verified are showing 'pending verification'. The odd thing is that the pending ones are the first alphabetically.
It's probably no big deal - I'll check in later on to see what G does.
UPDATE - in the time it took to write this, they all went to "pending". No access to things like crawl rate etc.
It may be because Sitemaps periodically looks for your verification files. Once you create a verification file, it's good to leave it up so that it can be found in the future. They did that so that if a domain changes hands, the previous owners will eventually expire out of the webmaster console.