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