Before getting too excited about GPS navigation using a laptop in your car, go sit in the car with your laptop and make sure the screen looks good enough for you. Unless your laptop has a reflective or transflective screen (and, unless I'm mistaken, current Apple laptops don't), you will have great difficulty seeing anything when you're outside during the day. No backlight can compete with the sun.
Assuming you're satisified with the display of your laptop, the next step is to pick the best software. Drivers are a red herring. Any GPS with a serial port will spit out standard NMEA. You don't need any drivers to use them. At most, you'll have to get a USB->Serial converter. You can get one for $20 from any number of sources. If you happen to find a USB GPS that has Mac drivers, that's great but don't let it be a deal-breaker.
The key is finding good software that can make use of the data from the GPS. Frankly, I don't see a whole lot for the Mac. Before you spend any time looking at GPS receivers, get the software figured out. Without software, there's no point in moving ahead with the hardware.
I've been using mobile mapping GPS hardware and software since the late 90s and I've used just about every combination of laptop/pda/standalone you can imagine. My first setup was a subnotebook (Toshiba Libretto) with a Sony Etak reciever and Sony SkyMap software. It worked okay in the car but I often had to shade the display to see what was on the screen. Not very safe. But I was able to load several different map/navigation/POI programs and switch between them as needed. Sony for driving maps because their color scheme was the only one I could read in the car. National Geographic for finding POIs. And some other program with a more current map database for finding streets in new subdivisions.
The next setup to get long-term use was a standalone mapping unit from Garmin. This system wasn't very flexable (crappy routing, low-res screen, minimal POIs, no detail maps, etc.) but it was compact and self sufficient. It would run all day on one set of batteries. Since I was using it on a motorcycle, portability and simplicity were the most important features. While I never got lost, I missed the extreme detail found in street level maps.
Now I use a combination of PDA and Garmin eMap. The PDA has a reflective color screen which is the best choice for outdoor viewing. Also side-lit for indoor/night time use. With a 512 or 1024 meg storage card, it can hold detail maps for most of the US. I keep it zoomed in fairly close so I can use it to navigate streets. On the other side is the eMap which shows an overview of highways and major streets. That lets me navigate surface streets on one display and highways on the other without having to make any changes. Both displays look great both indoors and out, day and night.
With decent mass-storage options available for PDAs (I've seen 2 gig CF cards already and 4 gig cards are on the way), they're really becoming a viable platform for mobile GPS use. Even without huge storage capacities, most of the current models can fit the data for several states in the built-in memory. There are quite a few software packages available so you can pick the one that best suits your needs. They're getting pretty fancy, too, with 3D navigation displays and lots of POI data.
