Wanted to rent a luxury car over the next few days, so I placed a bid w/Priceline.

I have never seen what Priceline emailed me back before, I quote the following,

"We searched hundreds of options to find your rental car, however we were not able to find a rental car company that would accept your price of $xx per day. Good news! During our search we found a rental car available at $yy per day. If you are willing to raise your price to $yy per day, you can get your rental car right now.

Normally you are required to change additional items (car type,location, or dates) in order to try your request again, however we are able to extend this one-time opportunity to you because of the type of rental car and dates you requested."

As I say, I have never seen this type of reply before. Due to the short time frame I had, I did go ahead and rent the car as it was still much cheaper than going direct or through Expedia, Orbitz etc.

Is this new or an exception as they claimed?

That's a counteroffer. Generally accepted response to counteroffers on biddingfortravel.com is to ignore them and continue raising your bid, as Priceline will almost always accept less than the counteroffer.

That said, there isn't a "legal" way to continue bidding and raising your bid for rental cars without waiting the prescribed 72 hours (like using alternate cities/airports for air tickets, or "free rebid" zones for hotels). Therefore, if you didn't have more than 72 hours before your rental start date, accepting the counteroffer might well have been your best course of action.