The AlertDialog widget used by the validation framework has a couple of navigation issues:
1) When it comes up, the "OK" button doesn't have the focus, but it appears to the user to be the only active page element.
2) Users can tab and shift-tab away from the dialog and enter data in form fields behind the bakground iframe. This is non-intuitive, and almost always undesired behavior.
It appears from the parent class, DoJo's Dialog.js that giving the button a tabIndex, setting it as this.tabStart, and setting initial focus to it should fix these.
Hard-core would be to trap all keystrokes and stop any key event that wasn't space or enter, but that's probably more of a hammer than is needed.