I am considering running MSBuild from a Powershell script by tapping directly to the MSBuild assemblies (as opposed to looking up MSBuild install path and starting msbuild.e
I was looking for this same thing. Following JaredPar's lead I found the following:
This is a how-to on making a cmdlet.
http://bartdesmet.net/blogs/bart/archive/2008/02/03/easy-windows-powershell-cmdlet-development-and-debugging.aspx
The MSBuild API is part of these namespaces:
Microsoft.Build.Framework
Microsoft.Build.BuildEngine
And the MSBuild documentation can be found here (this is more for completeness than in response to your question):
http://msdn.microsoft.com/en-us/library/wea2sca5.aspx