This problem is addressed in ArcGIS 8.1 Service Pack 1. See the Related Information section to download this pack.
When I perform a join and then use the Find tool, I get the following error:
This problem only happens when joining data from different data sources, for example, joining a dBASE file to a Personal Geodatabase layer. In this case, the different data sources are not taken into account when the find is performed.
Close and reopen the table window. The find tool will now work properly.