The mini-AB cable is optional. At least with OS2007 such cable won't help you much since there is lot of stuff you need to do by hand anyway. The mini-AB will only give you possibility of automatic switch to host mode. You can still switch modes by hand so such cable is not strictly needed.

Currently (with a lot of manual fiddling) it is possible to use just usb female to female adapter and original usb cable that came with N800. Then you can connect low power devices like flash disk, mouse (won't do much) or keyboard. For more power hungry devices you need powered hub or Y cable with additional power (haven't tried Y cable).