See question 3 and the followup responses at
<http://www.netbeans.org/servlets/ReadMsg?msgId=247950&listName=nbdev>

Basically, the idea is to implement marking for the core string editor with the
following requirements:
- If a value is invalid, show the mark in the property sheet
- Always enable the Ok button in the popup property editor.  There can be an
error dialog on ok (IllegalArgumentException/ErrorManager annotation by
setValue) implemented by the caller if there is a problem.  The Ok button
enablement shouldn't be updated on each keystroke.
- Make sure that the property marking is properly updated when the user cancels
and there is an invalid value but a valid value in the cancelled dialog.
- Make sure the in place editor continues to work as it did before

I will file a separate enhancement request for the rest of the core editors.