I have to throw in with Astimegoeson. Don't become involved with someone else until you are certain you are finished with your WAS. That having been said, there's technically nothing wrong with seeing someone even if you might still go back to your WAS. People who date (unless they have agreed to do so exclusively) generally understand that each may be seeing other people as well (and your WAS counts as another person).
However, while you may not be specifically trying to use them as bait, if you would still return to your WAS if s/he asked while you're dating someone else, you're essentially doing just that and most folks wouldn't consider a playing field that contained a spouse (even a bad one) level.
In addition, I think it's dangerous to base any kind of rekindled relationship on the feeling by the WAS that while they don't want you, they don't want anyone else to have you either. Once they feel safe that is no longer a possibility, it seems likely they'll walk away again.
