I have a VPS at rootbsd.net, and have been running 8.2-RELEASE with a XENHVM kernel with a patch to fix the 'do something smart' panic in if_xn. I fetched the 9.0-RELEASE source tree and built a kernel to try and it worked without any muss or fuss. I did the rest of the upgrade and it's working just fine, so far as I can tell. And there was much rejoicing.