First time Homebuyer's Credit question regarding repayment?

We took the $7500 credit on this year's taxes. Soon after my husband decided to join the military and we have since been trying to sell the house. The value of the house has gone down in a little over a year (~$4000 or so) and as such we have the house priced below what we owe for it. If it were to sell at a loss I understand that we wouldn't owe the money back, but what happens if we can't sell and he goes off to basic? If I then join him at his first station will the house no longer be a primary residence? If we rent it to defray some of the cost do we then owe the money back on next year's taxes? If we set up a "lease to own" for a prospective buyer in which they purchase in May of 2010 would we owe the money back? I've searched a lot for answers but the program is so new there isn't much discussion beyond "do I qualify". Thanks for any help.