There doesn't seem to be an "official" time webservice out there. There's a demo one hosted by Altova (makers of XML Spy) here that looks pretty good: http://www.nanonull.com/TimeService/TimeService.asmx. There are several other demo ones out there if you Google timeservice.asmx.
I couldn't build an enterprise production application that relies on a demo that Altova could take down at any time, though.
I know there are several official NTP servers out there time.nist.gov, time.microsoft.com (if you're not in the camp that believes Microsoft's time server is the first step in their attempt to control the very fabric of spacetime). I could implement or use an existing NTP client (this one looks good), but it's not a simple HTTP webrequest - it takes a TCP request and parsing the NTP time format. Since Microsoft.NET is all about XML web services, I shouldn't have to, should I?