Had it hanging-off a USB port of my Mac Mini for a few days, it was charging last I checked, but since then seems to have stopped receiving power from the USB port & gone totally flat.
I've plugged it straight into an AC outlet but still can't get it to power up, I then left it sitting their for about 20-mins but still no go. Any idea WTF the problem is?
It's pretty much stock config, I hadn't done much in the way of apps/hacks/mods since my last re-flash...

If the charging light show for a few seconds and then disappear.
You could try this:
http://everythingn9.com/death-by-bat...arge-nokia-n9/

You're ****ing kidding me, there's no way I'm dismantling this N9, too busy.
Is there any other fix/resolution?

The guide I linked does not say you need to dismantle the N9.

Yep, I see that now, still ****ing ridiculously complicated/time-wasting, **** that.
Thanks, but no thanks....

Just plugged it into the USB port of my rMBP, at least now the charging light is lit up & staying lit up.
I tried powering it on after about 20 seconds, but no response...

*EDIT*
Actually just now, about 20 sec later, it seems to be booting up...
*EDIT* 2
Now it's stopped the booting process & has a charging symbol on the LPM screen instead.
The charging light is now slowly flashing instead of being lit up solidly (think we're getting somewhere)
*EDIT* 3
Yup, just tried holding the power button once more for about 1-sec, it vibrated & continued the boot.

All good now, there must be something wrong with my AC charger...
*EDIT 4*
Unplugged from the USB port & plugged the USB cord back into the AC charger & now it's charging.
(Estimated time for charge completion much faster too, naturally)

"flasher -i" if you want it to charge without trying to boot.
"flasher --set-rd-flags=force-power-key -R" if you want to force it always to boot up without going to act dead charging mode,

Doesn't seem to have been necessary, thank God >.>