When a listing says "looking for back up" it means that the seller has accepted an offer from someone, but that they want additional offers on the back burner in case the contract they have pending falls apart.

In this market, where so many deals are falling apart because not as many people qualify for mortgages as before, it's not to hard to understand why a seller's wish list would include a fall-back position in case their offer fell apart. On the other hand, given the oversupply of property on the market (especially new construction), it would be hard for me to say that it is a realistic expectation. Unless the property was especially unique or had attributes which made it a must-have, I wouldn't bother waiting to see if the seller's current deal fell through -- It's too much of a buyer's market.