The illustration below was copied from the magazine Black + White Photography, January 2010, page 50. I felt is was important enough to copy because it shows the relative sizes of the current crop of sensors.
The two important sizes are the Nikon, Pentax, and Sony APS-C and the Canon APS-C. The companies named their sensor the same but they are not equal.
Nikon, Pentax, Sony APS-C: 23.6 x 15.7mm (1.5x crop) sensor area; 370.52 mm^2
Canon APS-C: 22.2 x 14.8mm (1.6x crop) sensor area: 328.56 mm^2
Just thought you should know.
Relative sensor size of most common sensors on the market.
NOCCC Beginner's Digital Photography SIG 