>I want to use one of the portb pins for a touch
>switch (human finger provides a high resistance path from the port
>pin to a ground terminal

I believe the pullups are modelled as a sloppy (non-linear) constant current
source, not as a resistance. This aside, the biggest problem with the idea
is that there is no proection from ESD (Electro Static Discharge). Unless
the user is going to touch a grounding surface before actuation you run the
*very high* risk of zapping your input pin.

Its probably best to insolate the input with a FET circuit. Protection
diodes on the input (with a current limiting resistor), and input pullup &
load resistors are also required. One or more capacitors may be needed to
prevent pickup or 60hz(50hz) hum as well.

One of the cheapest input switches I've used is a tactile dome. Its a small
pieces of dome metal, with 4 contact points. When placed over a PCB pattern
with mylar tape, it provides a tactile switch with good life and audible &
tactile feedback. The cost is something on the order of 0.05USD in hundred
quantities. The dome is electrically at ground and it covers the input
contact thereby providing complete ESD protection. These switches are
usually covered with a printed overlay in the product.

Regards, Dana Frank Raymond
dfr@icom.ca