I have a similar problem. Four datasets created the same way, but two of them are slightly different from the other two. They cannot all be loaded into the same Feature dataset. ArcGIS 9.3.1. The difference is in the Abscissa resolution and Ordinate resolution. In one pair the values are 0.000000, 0.000000; and in the other pair they are 0.000100, and 0.000100.
1) data brought in as XY event from excel table, UTM 11 NAD27, then exported as a FC into a FGDB, using the Data Frame properties: NAD_1983_Albers, Projection Albers, False Easting 0.000000, false northing 0.000000, Central Meridian -96.000000, Standard_Parallel_1 29.500000, Standard_Parallel_2 42.500000, Latitude_of_Origin 23.000000, Linear Unit Meter, GSC_North_American_1983, Datum - D_North_American_1983.
2. I tried importing the Coordinate system of one to the other, but it did not fix the problem. I also tried projecting one data set and importing the projection information of one of the others. This did not work either. Both times the Abscissa resolution and Ordinate resolution remained 0.000000, 0.000000.