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)