RESOLVED: The WS-Management service cannot process the request. The user load quota of 1000 requests per 2 seconds has been exceeded. Send future requests at a slower rate or raise the quota for this user. The next request from this user will not be approved for at least Z milliseconds.
We had a newly built Exchange 2010 SP1 server which was ready to go into production but kept throwing the error when attempting to use PowerShell. Two other servers appeared to be running fine.
The server had recently received a new SSL certificate using the Exchange 2010 certificate provisioning and assignment process in the GUI. Unfortunately the IIS service hadn't been restarted yet and the URL used for remote PowerShell was using a certificate which wasn't trusted or valid anymore.
A quick "IISRESET" on the server resulted in my fix.