While swapping over my locally built Baking Pi kernel.img files over the stock linux one on the SD card, I made a mistake and lost my linux one. I'd done an apt-get install update recently, which had installed an update to the raspbian version, but the download images at rasberrypi.org haven't updated to that one yet. So I didn't have a way to restore my stock kernel.
So I built a new one. On the pi. Which took several hours. But it worked fine. I was following http://elinux.org/RPi_Kernel_Compilation#Kernel_compilation for this.
I've also been having problems with Lesson 5 of the Baking Pi tutorial. Instead of blinking out morse code on the OK LED, my version just lit the LED permanently. Puzzled, I looked at the model answer for the lesson, and the code was very similar. I built and booted that version and got the same behaviour: solid LED. Most odd. To eliminate my mac toolchain as being the culprit, I built and installed Lesson 5 model answer on the pi itself by editing the makefile and removing the yagarto prefix from the makefile, i.e. changing
ARMGNU ?= arm-none-eabi
but this resulted in the same behaviour: a solid LED light. I've asked on the forums for help.