Silicon Valley Watcher says they have seen the tracks of the Google Browser. They don't show the full user agent though, so maybe their log analyzing software just got confused by the Google WAP Gateway or some such?
A few non-Googlebot Google useragents:
MOT-A835/70.20.18. MIB/2.2 Profile/MIDP-1.0 Configuration/CLDC-1.0 (Google WAP Proxy/1.0) Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1),gzip(gfe) (via translate.google.com) UP.Browser/22.214.171.124.140 (Google CHTML Proxy/1.0) Mediapartners-Google/2.1
It's also curious the Googlebot sometimes is pretending to be Mozilla. Or was it someone modifying the User-Agent?
Mozilla/5.0 (X11; U; Linux i686; rv:1.7.3; Googlebot/2.1) Gecko/20040914 Firefox/0.10.1