One of the reasons I haven't purchased a field monitor for my 5D Mark II is because the camera drops its HDMI output to 480p the second you hit the record button. Beyond the problem this presents for accurately pulling focus (because of the reduced resolution), this change in resolution is seemingly handled by every field monitor in a different way. Some scale the output accurately, others squash the image, and there is always a variable delay in between the time you hit the record button and the time the monitor refreshes with the new resolution. Monitor manufacture SmallHD is here to lay out the facts of DSLR field monitoring with their excellent video "Effective Pixels 101":
It's worth noting that not all HDSLRs drop their output resolution during recording (the 7D, for example, stays at 1080i). But as SmallHD points out in their article, we shouldn't hold our breath for a 5D firmware fix.