In case it wasn't already mentioned here, you can reflash the power kernel just by executing the already given command and replacing kernel-flasher with kernel-power-flasher. So the line reads:
apt-get install --reinstall kernel-power-flasher
This works perfectly and no more u-boot stuff on startup. will try next whether I can now reinstall the stock kernel (without u-boot) again.

EDIT: to be on safe side you could also reinstall the kernel itself and its modules in one go:
apt-get install --reinstall kernel-power kernel-power-modules kernel-power-flasher

EDIT2:
WIP: just deinstalled power kernel by using commandline tool kernel-power-deinstall. This worked great and it seems like the original kernel was flashed back again. Just powered down ....

EDIT3
yessss! current kernel: Linux-2.6.28-omap1! No power, no overclocking, no u-boot.
Now I'll get back to power kernel, 600MHz is way too slow ....

In short words: instead of fiddling around with USB and flasher and crude commands you can (simply) install or reinstall powerkernel and then go back to stock kernel.