As part of the Sitemaps verification process, Google specifies a filename that is composed of some sort of HASH - i.e. www.domain.com/google123456789.html that you have to create in order to verify it is your site. The IP address 64.233.172.18 came by to check that out ... but I then saw attempts to download www.domain.com/noexist_123456789.html from IP's 64.233.172.35 and 64.233.172.24. None of those resolve via reverse DNS incidentally, but all must be Google. Maybe I missed an earlier thread, but anyone know what the "noexist" is about?
Google is checking to see if your server 404's properly. If your server returns 200 for a nonexistant page, then the verification process cannot succeed (because you could then verify every site out there; been there done that: http://base.google.com/base/a/softplus/1006918/5976232315496969564)