NOTE* You will probably need to view this in HD. For some reason you can't see the grain very well in SD.
I recently saw a test done for a 7D showing the relative ISOs and the corresponding noise amounts. After recently hacking my 550d with Magic Lantern, I decided to run through the 49 (!) ISO settings on it and compare the noise levels as well.
Notice the noise levels on any ISO with an even multiple of 160 (until 1600). This a phenomena I'm not going to try to explain. Any ideas?
Also, notice the similarity in the noise levels of 1250, 1600, and 2500.