Fast Five hasn't even been released yet, but Universal has already hired a screenwriter, Chris Morgan, to pen the screenplay for Fast Six (he also wrote the screenplays for the 3rd, 4th and 5th installments of the franchise).
No word yet on what the plot will be; but I'm sure it'll involve fast cars, women, and lots of action :)
Collectively, the first 4 films grossed almost $500 million domestically, and close to $1 billion cumulative worldwide; so, it's obvious why the studio continues to crank them out. Although, I should note that each successive installment has grossed less than its predecessor.
Fast Five will be in theatres this Friday, the 29th. From all I've seen thus far, it's not a movie I'll be rushing out to see; I saw the first 2, and they were more than enough for me. Maybe I'll catch it when it hits Netflix's streaming library.