Bluetooth keyboards work with all existing Maemo devices and will work with Maemo 5 devices also. Before purchasing a Bluetooth keyboard check its technical specifications though.

Bluetooth keyboards come in two varieties: HID (Human Interface Device) and SPP (Serial Port Profile). Bluetooth HID keyboards require no drivers and easily and immediately work with Maemo devices as soon as they're paired. Bluetooth SPP keyboards require drivers, and while they often can be made to work, it's not nearly as easy. Thankfully, most modern Bluetooth keyboards are of the HID type.

I wouldn't count on that. Bluetooth is designed for low power, mobile use, while USB keyboards are generally designed for desktop computers. In the end, neither should draw too much power though.