I went to the Canon Paris expo and had a quick play with this lens. I asked about the zoom lock feature, which is not actually a zoom lock as far as I understood the response. Rather it locks the available zoom range to the sensor size for APS-C or H. I used it on the 1DS3 shown in the pic. It was not fully circular at the widest setting. I asked if I could try it out on my own camera, or use my CF card, but they said no:(
You can see the C and H on the lens in the attached pic. There is a lock on the other side you set to lock the range to either one.