If the resistance between the data pins is over 200 Ohms, the device will not detect the chanrger as a high current source and therefore limit the current to 100mA. That the current it is allowed to draw from a PC USB port without requesting more power.
Since most wall chargers do not yet follow this new rule, they will only charge very slowly. This is the case with most wall chargers I tested. My solution was to use an USB extension cable and shorten the data wires. This doesn’t hurt at all, even you still would connect it to a computer.


Now, about the charging current:
I created my own external USB battery pack and did some measurement of the charge current. To my surprise, the charging current is highly dependant on the voltage supplied at the USB port. Since I wanted to protect my N900 and N950, I lowered the current to 4.1V. This made the current to go down to 300 mA max, even when the phone battery was completely empty. When I raised the voltage to 5.0V, I now can get 800 mA max. One could go up to 5.25V which is the maximum according to the USB specifications. Since I want to stay on the save side, I will stay at the 5.0V.
This will take maybe 1h for 70..80% and then slow down since the battery is almost full.


Beside of all, the charging electronics in the phone will limit the current. This means you can use any USB charger you have. How ever, if you supply a too high voltage (>5.25V), it might get damaged!