Tim Kent at Adatis did a nice writeup on how to work with the "well-known properties" Name, Description, and Person Responsible of a PerformancePoint Monitoring first-class object (a KPI, a Scorecard, a Data Source, etc.) when calling the PmService web service.

I've always wondered why they're "well-known" if you have to memorize a GUID to work with them... "well-known if you have a good memory", perhaps. It should be easier than that, and I've entered a usability bug against this for the next version.