Just walking away may seem like the easiest way out, but it may in fact make things more complicated for you. If you have a HELOC, I do believe a short sale is usually worth the hassle. At the very least, a short sale gives you the opportunity to negotiate with your lender up front - and, foreclosure is always there as an option, but a short sale isn't.
An “approved” short sale is a short sale where the short sale lender has already communicated to the seller what they will accept as an offer for the property.