Also, you don't need to use an external data group on 10.1 or higher. You can use an internal string data group with the name=value pairings. In that case, you can update the data group via the GUI or iControl and not worry about reloading the external file.
If you do use an external file, I don't believe it gets reloaded ever automatically if you edit the file on the CLI. You'd need to modify the data group via the GUI or iControl. Here's an example from Joe for forcing a re-read of the file for an external data group via iControl:
http://devcentral.f5.com/wiki/iControl.PingExternalClass.ashx
Aaron