IME, the kernel booted the phone right up, I could hear a faint click sound as I normally do when it boots up, and felt the subtle vibration after the "Android is upgrading" thing had finished (I left it on for a while). The capacitive keys lit up as well, but the screen was black through the entire boot cycle, it didn't once render any image on screen, just black.
After returning to the recovery, to flash the older version and then rebooting, I noticed that ALL the Android system settings were wiped out, my apps and data were in tact, but it was behaving much like a first boot, with the guides kicking in for folders and the app drawer.
Really strange, but I made a nandroid first, so all is well again.
Hope this helps, I didn't manage to get any data (last_kmesg, etc) regarding what might have happened.
The new app works really well!