Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
If it's just a playlist you deleted then your songs will still be in iTunes. If you're talking about restoring the actual playlist then you can't as the undo button is greyed out.
 
Stay tuned, hopefully someone else will have a solution. So far I created 2 playlists and deleted both and the undo button stays greyed out and I couldn't recover them.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.