During a project where there should be different types of teamsites, document and meeting workspaces, we had to cope with a strange problem in Sharepoint. What we wanted to do, is to make a duplicate meeting workspace (the MPS directory in the layouts directory), and adjust it to our needs. This way, we kept the original and didn't have to worry about overwriting the original template in a future service pack.
However, after registering the site definition and a IISRESET of the server, the new workspace could be created, but some webparts displayed an error telling us that they would only work on a meeting workspace site! After some research it turned out that the MPS site definition has a hardcoded ID of 2. Any other number will give these errors. The result was that there could only be one meeting workspace template at a time. Unfortunately...
Weird that there isn't any note on this issue in the SDK.