> Will a ULN 2803 (8 darlington drivers) protect its inputs from back-EMF?
> In the case of a PIC driving a 2803 which drives the stepper motor coils,
> is there any danger of hurting the pic?

A 2803 has bleeder diodes to the Vcc, which is the only function of the Vcc
pin. Don't forget to connect it, otherwise things will work normally but
only for a short time. The bleeder diodes protect the 2803, the circuit
driving the 2803 won't notice any spikes.

> 4 PIC pins will drive both motors, with 2 XORs and 4 Inverters to derive
the
> other signals for bi-phase (uni-polar) drive (0011, 0110, 1100, 1001)
from
> (00,01,10,11).

If you can spare the pins use 2 x 4 for the motors, driving one 2803
directly.

> Should I stick a 0.1uF or <what value> uF cap on each of the 4 PIC pins?

No, what for?

The steppers work best on 12V.
I make small robots (StepBots) this way.
12V accu pack (10 penlight nicards), PIC, 7805, 2803,
add proximity switches or IR-reflex sensors
and you have a very simple (low performance) robot.
see example 3 (I think) in the JAL manual
( www.xs4all.nl/~wf/wouter/pic/jal ) for a
simple software example.

success,
Wouter.