Deletion of the first post of a thread triggers deletion of the entire thread; undeletion doesn't do the same, as it should to be consistent (example: http://strategy.wikimedia.org/w/index.php?title=Special:Log&dir=prev&offset=20100906064357&limit=4&user=Nemo+bis ).
You can argue that undeletion will be less common that deletion (for vandalism etc.), so I'm marking this as low priority.
Version: unspecified
Severity: enhancement