I have had other reports that power kernel does not work. I suspect this happens because the old old kernel used on N900 does not fully initialize the hardware itself. It relies on NOLO to do most of it. Anything that u-boot touches has the potential to be in a "wrong" state when the kernel boots. This can cause all kinds of problems. For example, it is the reason why usbtty is disabled. There are two ways to fix this, the right way, and the work-around which is necessary when dealing with vendor kernels:

The right way is to fix the kernel so it will reset hardware from an unknown state without crashing. The new new kernel used in MeeGo does this, and I have not had any problems with it.

If you can't fix the kernel, then the workaround is to make sure u-boot puts all the hardware back exactly as it found it. This can be tricky if you are trying to boot from MMC...