As Cory stated, It never hurts to ask. I've been trying to sell a property for several months and finally decided to rent it. Best thing I could have done. I'm cash flowing $550/mth. I'm going to keep renting it until the values start going back up and then put it back on the market.
Renters could make a home difficult to show, and there are tenancy laws involved, both of which could make a home less attractive to a buyer. It can't hurt to ask though. You might even get a lease if the seller decides it's in his best interest to take it off the market to rent out. It really depends on why the seller has the home on the market in the first place.