Barry Tousley, an SDET on the PerformancePoint Server team has released a tool to help you do PPS planning data  migration.

You can find it here:

http://www.microsoft.com/downloads/details.aspx?FamilyId=FA1C6CAD-45BD-4E5C-A257-4BBA84FA4224&displaylang=en

Information on the tool itself is here