I haven't seen a copy of the filing, but reports indicate that Netflix patent #7,024,381, awarded yesterday, is the basis of the patent infringement lawsuit.
Here's the abstract for the patent:
According to a computer-implemented approach for renting items to customers, customers specify what items to rent using item selection criteria separate from deciding when to receive the specified items. According to the approach, customers provide item selection criteria to a provider provides the items indicated by the item selection criteria to customer over a delivery channel. The provider may be either centralized or distributed depending upon the requirements of a particular application. A "Max Out" approach allows up to a specified number of items to be rented simultaneously to customers. A "Max Turns" approach allows up to a specified number of item exchanges to occur during a specified period of time. The "Max Out" and "Max Turns" approaches may be used together or separately with a variety of subscription methodologies.
Netflix was also awarded patent #6,966,484 in November, 2005 for a "mailing and response envelope."