Arne Hess of the::unwired has confirmed that Windows Mobile 5.0 will natively support Bluetooth HID profiles. If your Bluetooth keyboard supports HID profiles, then it should connect to your WM5 device without you having to do any driver or software installation.
Unfortunately, it’s not all good news. The Windows Mobile 5.0 implementation does not support languages other than the one installed. In other words, don’t expect to use a french keyboard on an english version of Windows Mobile 5.0.