A quick post to describe how to build a Xamarin iOS project from MSBuild on a Windows machine. You still need a mac available but this approach means you can create an iOS artefact from a Windows TFS build agent.
On the command line run:
msbuild YourProject.sln /p:Configuration=AppStore;Platform=iPhone /p:ServerAddress=MacIpAddress /p:ServerUser=MacLogin /p:ServerPassword=MacPassword
When this is running keep an eye on the mac as it might pop up a message box or two on the first run.
After the script completes you should have a new IPA file in the bin\iPhone\Ad-Hoc directory of your iOS project on the Windows machine.