Why does Internet Explorer Mobile 6 use a layout viewport of 974px?
974 seems like a suspiciously precise number.
The number is chosen for 1024x768 monitors, which were the de facto standard for a long time. 1024 - 974 = 50, which allowed for 25px margins on each side.
For 1024×768 screen resolution, the approximate visible area is 974×650.
Many grid systems use that width, such as the following: