dsPIC33 and the upper half of PORTB
Ok, I have had a response on the Microchip forums...

Apparently the pins associated with the upper half of the PORTB register are shared with the JTAG system.  JTAG defaults to on, and can be disabled in the configuration fuses.  In MPLAB C30 it is:

_FICD(JTAGEN_OFF)


to disable JTAG.

Once this is done those pins all work perfectly.