No, powered hubs are... Well, powered, transparently for host device. At least if they're not pushing current to uplink, which they shouldn't do.

BTW, if you have missed it - max current N900 can give to USB devices is ~200 mA, while "normal" USB specification says 500 mA as max (for 2.0). It should be quite easy to judge, what may take up more.

Completely unrelated to current, but if you start to get "hub times out" messages in dmesg (kernel messages log), for no obvious reason (ijn situations, where everything should "just work") - disconnect everything, modprobe -r g_file_storage, then modprobe g_file_storage again, and try from scratch. Of course, it isn't related to any external hub, those messages are about N900's internal hub and its state.

/Estel