The point feature class had M values in it. While not a problem in 9.3, ArcGIS 10 looked at is as multipoint feature class.
So, I assume, that if a single point feature class has M values it is defined as a Multi Point Feature Class somewhere, although it does not explicitly say MulitPoint Feature class in the Properties Dialogue. Apparently the M value did not cause any problems with 9.3 locators or, the M value in 9.3 did not change the feature class from Single Point to Multi Point.