Yeah, basically the N900 want to negotiate a current with the charger. If the charger isn't made to do this, the Nokia refuses to charge. It could have assumed that any USB charger that doesn't support a negotiation protocol follows the USB standards. It doesn't.

The good news is, with a hacked cable adaptor you can probably use the battery solution you have now. As is the case with old "dumb" chargers.

More about it here: http://talk.maemo.org/showthread.php?t=38230

I also assume that this is the issue with the notebook - it can't negotiate while in standby.