There is a "handshake" of sorts going on. USB cables have four conductors: a power pair (5v/gnd) and a signal pair (D+/D-).

Shorting the signal wires is the (standard?) way to signal the presence of a dumb charger capable of sourcing at least 500mA. I'd wager that the N900 uses this to decide if it can sink charging current from your external sources. Cut open a spare USB cable, short the data lines (inner pair), and see if your charging problem is fixed.

If you have a multimeter, you can confirm if the official Nokia charger does this internally by measuring the resistance between the data pairs. The charger can be unplugged prior to probulation.

Sadly, I can't test any of this myself, 'cos I'm still waiting for Dell to ship my phone.