Hi there,
Just for the record: it's not the combo that needs to be filtered. It's the associated DataSource (SqlDataSource or LinqDataSource) that needs a WHERE clause.
Here's an example:
storing the login name in my personal database
It shows how to do it with handling ItemInserting. In your case, you need to handle the Selecting event of the data source and feed it the user name.
Please let me know if this helps. If not, provide a bit more information (about the exact DataSource you're using) and I'll cook up an example for you (might take some time).
Cheers,
Imar