Problem: The validator callout doesn't appear - the server side validation does prevent the insert from happening, but no message is displayed to the user!
This is a known issue on the controls page known issues section:
The callouts do not currently display automatically after a server post-backI suppose it makes sense for it not to work, but its a bit of a shame that it doesn't.
and will only work for custom validators which utilize client-side validation.
Even after a post-back the callout will display when the form is re-validated
when a postback is attempted again.
So Validator callout will only work for client side validation (works great for all the other validators, just not the custom validator)!