I won't be surprised if the franchise goes to 7, 8, 9, 10 and beyond... I haven't seen Fast Five yet... didn't even see Fast & Furious 4.

From Deadline:

The release date for Fast & Furious 6 is Memorial Day (May 24) of 2013... The global blockbuster franchise... reunites its ensemble cast "for their most dangerous adventure yet", according to the studio. Neal H. Moritz and Vin Diesel return to produce, and Justin Lin is lining up to go behind the camera for the fourth time in a row.