It sounds like the head honchos over at Avis realized that this kind of short term rental was the next big thing, and they've gone after Zipcar with an acquisition in the hopes they'll now also gain them access to the technology—and to the younger folks—available through Zipcar. That means Zipcar will probably hang around as a kind of subsidiary, so the Avis website will not necessarily be where you’ll find your weekend rental later in 2013.
In fact the big cheese over at Zipcar was on CNBC last week to share that message, and he mentioned that he thinks his company will still be able to make their own decisions. One such decision might just be a membership-free approach to their rental car model, as the company has been testing out something called the "Access Plan" up in Canada. Basically this allows users to skip some of the traditional Zipcar rules and regulations—and fees—to just get in and drive. Right now it's just a weekday option, but things might expand and change now that Avis is scooting into the driver's seat.