My bro (rCX) once calcualted one centimeter to be about 40 pixels. This makes sense because that would make one inch about 100 pixels, exactly.
However, this may vary depending on the computer; I'm not really sure. There's a wikipedia article on the whole thing
here.
Here's a quick way to check: This picture (from the wikipedia article) is 200x200 pixels:

Copy it into MS Paint or something and print it out. If it really is 100 pixels per inch, the square should be 2 inches by 2 inches.
--j_k