WebPart: Register tag is added automatically to .ASPX file when you add PPS web part

This observation can be used in other context.
Suppose you've have a .ASPX page in SharePoint 2007 that has Performance Point Dashboard web part. To uses the SharePoint site on a client machine that does not have PPS, remove
%@ Register TagPrefix="WpNS1" Namespace ="Microsoft.PerformancePoint.Scorecards.WebParts" ....

If Client machine has PPS installed, and when you add the PPS web part to your page this entry is automatically added.