My n900 has never turned on when plugged in and i've had it for about a year and a half.
I usually have to give it about 10-15 minutes of charge time.

I did not notice such behaviour (not turning on while on charger) with standard/power kernel only.
But since I have u-boot, it is something similar (exactly the same):
the phone does not turn off completely while plugged to charger. It shuts down the apps and the OS completely and display goes black, but keeps itself a bit alive somehow, because then I do have this problem also: I am not able to turn it on. In this stage u-boot comes up, kernel loaded, dots and again u-boot, ...
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tested with powered off device, plugged, tried to turn on --> nope, with u-boot from Matan and KP47
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So just unplug, wait until u-boot showed up, display goes black and after a few seconds phone is completely off,
You may check state of "offness" by pressing power button and check LED. If it comes white, release power button: the phone is off.

Another approach I use always (not only since u-boot) is:
before I plug the charger, I always turn on my phone and immediately as I see the boot loader (u-boot, standard kernel) -or the white LED goes off- I plug the charger. I do this as a manner, because after plugging the charger it takes too much time to wait for boot loader finished, so I may start the phone again. Maybe this helps out a bit ...

Mine has done this since installing kp v47. I just turn it on and when the white nokia screen appears, plug it in and it starts.

had same problem, since latest cssu update and maybe kp48

i have the same LED problem too....

Mine has been like this ever since i can remember, though i always install PowerKernel and a bunch of other stuff right after any reflash so i dunno if it's somthing i install that changes things or if it is always like this even with stock firmware and nothing installed.

There is a bug, but it's been closed due to WORKSFORME:

https://bugs.maemo.org/show_bug.cgi?id=6806

Considering there's a fair few of us, how do we get it reopened?

Managed to reproduce this problem a long time ago with kernel power 46
So probably yes this might be some issues with kernel-power. May I know what version of kernel-power you have? Mine is currently BFS so far no problems at all.

I have the same problem. KP intalled at the time of purchase N900. With the KPv49 problem persists.

I HAD the same problem, but a few weeks ago it suddenly disappeared.