After additional debugging I located the culprit. It was an innocent looking 4 TB drive that got plugged in at the same time the video driver was updated.
Plug in a 4TB USB drive and the system will not boot. Plug it out and life is good. Hence it's a BIOS bug.
I have a M4800 with the nVidia video controller running Windows 7 x64.
I applied the "urgent" A07 video driver update and it introduced an interesting boot problem if the system is used with the Dell advanced docking station (that supports USB 3 ports).
The system will only "boot" to the Dell logo and stay there. Hard drive activity stops after the telltale pattern just before Windows normally switching the screen to show its boot animations.
This is completely repeatable and downgrading drivers made no difference to the situation. Presence of external monitors (0, 1 or 2) made no difference either. The only way to get the unit to boot is to power it off, detach, boot into windows on battery power, and then reattach to the docking station.
Interestingly the "old" advanced docking station that I have left over from my M4600 works fine, though it doesn't support USB 3 ports.
I would appreciate any help that I can get from Dell, before we get into hardware swaps that are painful and expensive on everyone.]