Well this just gets weirder. I've really not a clue what's going on.
I moved my desk and PC around, plugged in the mouse and keyboard directly into the computer, and booted 3.4.0-1. Everything worked fine, which wasn't particularly unexpected. So I moved them back to the monitor's ports (possibly not the same way around as they were originally, but there are only 2 ports on the back and 2 on the side). They carried on working. I rebooted. Still working. I've given the power a full cycle and it's still fine. I didn't even touch the USB cable between the monitor and computer, although moving things around might have nudged it a bit I suppose.
Given that this problem has been bothering me for several days, but only
with 3.4.0-1, and with other kernels the side-ports were frequently used for hard-disks and SSDs, I'm stumped! Is 3.4.0-1 just more sensitive to dodgy USB connections or something? That seems unlikely...
I don't think I'll close the bug until tomorrow, when I'm satisfied that it all still works after a few hours sleep with flat capacitors.
For what it's worth, the monitor is a Dell SP2309W driven by an nVidia GeForce 9800 GT (Nouveau), and an ASUS P6T Deluxe motherboard. The mouse and keyboard are pretty bog-standard.