Again, it's all about appearance rather than actual substance. I agree that bringing a device which is
obviously a GPS is just asking for it. I'll bet, though, that few screeners or flight attendants would
recognize a GPS PCMCIA card, or the external no-display GPS receivers sold for use with mapping
software. You could always record the data while using your laptop for something else, then
plot your flight when you get home. I know, it's not the same thing, but...

Either way, it's going to be tough to get a usable signal without being very obvious you're doing
something weird (mommy, why's that man holding that piece of plastic on the window?)

I've done this before, only with a sectional and on a turboprop from Abilene to DFW. We were low
enough to use the same sorts of checkpoints that'd I use in a 172. I was amazed
by how fast we were moving once I used a chart I was familiar with. :-) :-)