Gmail is detected as a virus according to Microsoft's virus scanning software Windows Live OneCare. ZDNet Blogs report is available at
http://blogs.zdnet.com/Google/?p=386

including link to Windows Live OneCare Anti-Virus Forum thread with 49 replies:
http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=909837&SiteID=2

AV vendor Trend Micro sees this virus "BAT/BWG.A" as BAT_BWG.A listing several alias names:
http://www.trendmicro.com/vinfo/virusencyclo/default5.asp?VName=BAT%5FBWG%2EA

Technically the file detected erroneously as a virus is mail[1].htm located in Temporary Internet Files folder (it seems people reporting are using MSIE).