The Wide Fit feature requires one of the following before it can enable:
DTV = 1080i signal
Component = 1080i, 1080p
HDMI = 720p, 1080i, 1080p
You might want to make sure your cable box is outputting one of these specific signals based on the connection and the box's internal menu.
Wide Fit will not work on ATV, S-video inputs, Component at 480i/p, or through the PC inputs.
I have a Samsung LN46B750 and under the picture size menu, the wide fit and screen fit options are grayed out and can't be used. Why?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic