Thanks for the replies.
Yes, I've heard that not all lenses collect this data. I use a Canon 1D Classic and a 10D, and the lenses I use the most, also made by Canon, are the 35mm f/1.4L, the 50mm f/1.4 and the 100mm, f/2.8
I also have a Speedlite 580EXII which I use for flash as well as focus assist. Seems like this data would be somehow extractable from that device, too..