Until I found the following post on stackoverflow.com:
Here's a list I've compiled for my own reference, along with some of the legal values that can be used. Note that these are passed into MSBuild using the /p:<PropertyName>=<Value> syntax.
DeployOnBuild True False DeployTarget MsDeployPublish Package Configuration Name of a valid solution configuration CreatePackageOnPublish True False DeployIisAppPath <Web Site Name>/<Folder> MsDeployServiceUrl Location of MSDeploy installation you want to use MsDeployPublishMethod WMSVC (Web Management Service) MsDepSvc RemoteAgent AllowUntrustedCertificate (used with self-signed SSL certificates) True False UserName Password SkipExtraFilesOnServer (leave existing non-conflicting files alone) True False
The property I was missing was "SkipExtraFilesOnServer". I hope this helps somebody else. I really wish Microsoft would make the documentation for msbuild more complete.
Also of interest my be this article: http://blog.allanglen.com/2010/12/using-tfs-2010-team-build-web-deployment-with-iis-6