I was curious if there's a .Net API that would allow me to identify what updates are pending for "Windows Update,"
failing that, is there a windows powershell command that can get it?
The Windows Update Agent API may be what you're looking for:
http://msdn.microsoft.com/en-us/library/aa387287%28VS.85%29.aspx
It's a COM interface (not .NET native) but you can consume that from your application.