PB is part of Fox's clever master plan to make 24 look good by comparison when it finally returns. 
Looks like this will be PB's last season (and the story can't stand to be stretched any further anyway):
PI Feedback PB Ratings.
Fox has some decent shows in its fall lineup; unlike NBC, it won't be desperate enough to hang onto a show that's hovering around 7M.

Looks like this will be PB's last season (and the story can't stand to be stretched any further anyway):
PI Feedback PB Ratings.
The two-hour season-premiere of Fox’s Prison Break was a disappointment at 6.48 million viewers and a 2.6/ 7 among adults 18-49 from 8-10 p.m. Compared to it’s already below-average year-ago opener on Monday, Sept. 17, 2007 (Viewers: 7.51 million; A18-49: 3.3/ 9), that was a loss of 1.03 million viewers and 21 percent in the demo. Here is the half-hour breakdown:
Prison Break (Fox)
8:00 p.m. – Viewers: 6.16 million (#2), A18-49: 2.5/ 7 (#1)
8:30 p.m. – Viewers: 6.06 million (#2), A18-49: 2.5/ 6 (#2)
9:00 p.m. – Viewers: 6.65 million (#3), A18-49: 2.7/ 7 (#1)
9:30 p.m. – Viewers: 7.03 million (#1), A18-49: 2.8/ 7 (#1)
Fox has some decent shows in its fall lineup; unlike NBC, it won't be desperate enough to hang onto a show that's hovering around 7M.