I'm happy to announce that we've written a Unity3d build runner plugin for Teamcity (as we use both here at Mind Candy) and we have made it available at github for everyone to use: https://github.com/mindcandy/Teamcity-unity3d-build-runner-plugin
More info is on the web site / in the Readme but hopefully this provides a simple way to build Web / Windows / OSX Unity Players from Teamcity.
There is a prebuilt zip file at https://github.com/mindcandy/Teamcity-unity3d-build-runner-plugin/downloads if you don't want to build it yourself.
This is something we are actively using and will develop further, but other contributions are very welcome also.
Announcing a Unity3d build runner plugin