For those still in trouble:
I have just bought a new spare battery which holds almost full charge already.

THE USUAL FLASH MODE AND REFLASHING JUST WORK.
THE CURE IS VERY SIMPLE: GO GET A FULLY CHARGED BATTERY AND THEN DO THE FLASHING.

I'm not quite sure yet whether uboot is gone witht the flashing, but anyhow the n900 wakes up from coma.

IMHO three things are involved in this non-booting issue:
1) New version of kernel-power (v44): some blame the problem on the kernel. I beg to differ. According to those who flash successfully things are just fine.
2) Uboot: is it really usable at this stage? It's open to unlimited ways of configuration. What works and what doen't, I don't know. So next time if I am about to upgrade the kernel I will definitely check to see if the bootloader is well-behaved.
3) The flashing process: the most critical part is not whether you press "U" or how to plug in the USB port, BUT the battery level at the time of flashing.

I hope the troubled can try out the solution and hope that works for you.

I still think this is not a problem of the kernel but the implementation of uboot in this instance. Yet I would like to express my gratitude to titan and matan for their hard work which just aim at making our n900 experience better.