The USB spec says that you can't pull more than 100mA unless you've asked for it.
Then you can have up to 500mA, *if* the host root hub says agrees.
On the other hand, if the computer says no, you are limited to 100mA.
That said - 100mA is a reasonable trickle charging current for a low-power device.
My GPS has a 1400mAh cell - so 100mA will charge it in 14 hours.
Unfortunately a lot of USB root hubs provide exactly nothing when the host PC is switched off - and most laptops do this, for fairly obvious reasons.
I'm not sure if any powered USB hubs currently exist that have suitable chipsets to automatically provide 100mA to downstream ports when no host is present, but the standard does permit it.