Hmmm.. just for curiositiy's sake, I just tried the power button (with the phone switched on) and lo and behold, it does nothing... How bizarre..

That doesn't explain how reflashing it got it to boot, other than I flashed it in R&D mode as I read that in R&D mode the device powers on as soon as a charger is connected or the battery is inserted. However, this did not initially help - although it did cause the device to power up, it did the couple of scrolls of the booting circles before stopping again..

It was only when I reflashed with -R that it worked - I flashed from a mac but then connected it to a PC (I was going to try to recover the eMMC data from a ubuntu Live USB install) and it did the booting circles thing for quite a while (I thought it was just going to be stuck doing that forever), and then surprised me with the welcome/location setup screen...

Odd.. maybe the power button was stuck down, and so the time it took to start booting and then stop was the equivalent on how long it would take if I held the button in while booting to shut it down..