We like the ISO war that is shaping up.  Nikon fired the first shot by releasing the Nikon DS3 and Canon was quick to fire back (yes that was a joke) with the EOS-1D Mark IV, which boasts a 16.1 megapixel sensor and an ISO up to 102,400, which coincidentally (or is it?) has the exact same ISO of the DS3.

Read the article at Digital Photography Review: Canon updates pro line with EOS-1D Mark IV

To learn about ISO speed (AKA Sensitivity) read the page on Film Speed from Wikipedia