According to this post:
If a post was deleted by users other than its owner, the owner's undelete vote will no longer instantly undelete - it will be counted just like other users' votes to undelete.
Note that if a moderator participated in the deletion, only another moderator can undelete.
This doesn't seem to be the case with posts deleted via the review queue. For example, this post (Unix 10k only) was deleted by review, but then the answerer undeleted it single-handedly: