>> Once the data is submitted to the back-end it is too
>> late to do anything about it.

Why's that? The back end can very well tell you if there is something wrong with the data. Also, if the middle tier handles only data-data and not meta-data, the back-end would handle all of these issues internally. This is one of the reasons why users shouldn't be able to mess with PKs. But this is a different story again, and with this comment I propably just startet a monster-thread...

Markus