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.
Thank you very much for developing and publishing the plugin! I've update our plugins page with the link.
Your feedabck on how easy/hard it was and what is to be improved in our plugin writing "tools" is welcome!
You might consider listing TeamCity versions (and may be Unity verisons) which are meant to be suported.
PS you can probably update "Installation" section of the readme.