If an unlicensed client connects to a Terminal Server for the first time, the Terminal Server issues the client a temporary Terminal Server Client Access License (CAL) token. After the user has logged into the session, the Terminal Server instructs the License Server to mark the issued temporary Terminal Server CAL token as being validated. The next time the client connects, an attempt is made to upgrade the validated temporary Terminal Server CAL token to a full Terminal Server CAL token. If no license tokens are available, the temporary Terminal Server CAL token will continue to function for 90 days. The license is stored in the client's registry.
Thirty-two bit RDP clients store their license under the key HKEY_LOCAL_MACHINE\Software\Microsoft\MSLicensing.
Unfortunately, whether you have a proper TS CAL or not, the license information on the Mac needs to be reset from time to time. And in the first version of Microsoft’s client for OS X, this could be done easily by removing the contents of the folder:
/Users/Shared/Microsoft/RDC Crucial Server Information/