If you have the assets to buy another house, your current lender will not allow you to do a shortsale. In order to get short sale approval you need to show that no matter what you do, you cannot pay your current mortgage payment.
A better answer for you might be to do what we call a short re-finance. We are seeing some success in this area (although I do not do them myself, I can refer you to someone who has done a lot of them this year). What happens in this scenario is you show:
1. You would default on the loan the way it is currently structured,
2. You owe more than the property's current value,
3. If you only owed what your property was worth and could get a fixed rate mortgage you could stay in the house and make the payments.
If this is true, then this might be a better solution for you.
Contact me directly and I can help you work through your options. I specialize in helping people make the right decision when they appear to be a no win situation. I wish you the best of possibilities and look forward to speaking with you.