> Simple question really - does the tmro register when being
> clocked by an external source continue to increment even
> when it has rolled over ?

Yes. Everything that applies to TMR0 (eg pre-scaler, IRQs) etc
is just the same whatever the source. Note that you don't need
an ISR (interrupt service routine) to pick up the FF to 00 rollover.
Whether you have interrupts enabled or not, T0IF still goes high
and can be detected manually by polling it eg

checkrollover btfss  intcon,T0IF
                        goto  checkrollover
                        bcf    intcon,T0IF      ;clear it for next time

>  when should the weak pullups be used

Generally used for things like keyboard inputs, switches etc. They
aren't really suitable for inputs via long leads, which should have a
lower value resistor (a few k) to minimise noise pick-up. For input
sources like CMOS or TTL gates you don't need them. Turning them
off during SLEEP will save power

Search for "pullups" at www.piclist.com and in the mail archives,
it's been discussed a few times

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