Joikuspot adds a constant 300-500mA drain. Charging from a USB host would at best keep the battery level from falling.

Add 3g on top, which can consume 200-600mA extra, and you'd be losing charge fast.

The maximum charge from wallcharger is 950mA, and connecting it makes N900 draw 100mA extra since PR1.3.

The power balance is thus -300-200-100 + 950 = 350 at best and -300-600-100 +950 = -50 at near worst.

Using usb instead of the wifi is a drop of 300mA in consumption, but USB host is limited to 500mA.
Thus you have a range of -200-100+500 = +200 to -600-100+500 = -200