My friend's problem with the Boron not booting (and flashing charger light yellow rapidly) turned out to be power related.
He was using 3 x 1.5V Alkaline batteries to power the VUSB pin. The voltage had dropped to 4.4V total after much usage and the Boron refused to boot (with the yellow charger light blinking fast).
I've easily reproduced it with a lab supply. Mystery solved... (Occum's razor intact).