Hi Kim, The correct answer depends on what kind of listing the property is.
If it is a standard, regular, or equity listing, or an REO, ( Bank owned.) then a status "B" ( Backup offers.) or "P" ( Pending.) is fairly likely to close escrow, without being open to "accept" a backup offer - even one at a higher offer, or using all cash.
On the other hand, if the "B" or "P" status is for a short sale, especially a relatively new short sale listing, meaning less than 30 days on the market, then it has at least a 50% likelihood of the initial sale falling apart, plus a likelihood of the price becoming higher, when it does come "back on the market".
In this time of substantially reduced "inventory", it might be wise to keep an eye on short sales that haven't been on the market for a long time, even putting in a backup offer on any that you like. Chances are pretty good that it might come to you, as the next buyer, in line.