I've been asked several times for a solution that allows one to have wireless access in a vehicle. On a beautiful sunny day in Philly the need for this capability is immediately apparent to me :-)
One of my customers, a private detective, uses a Linksys wireless broadband router ( p/n WRT54G3G-ST). You can power it with the included car adapter or use an inverter in your vehicle. Just take your Verizon or Sprint EVDO card and follow the directions. It's reported that it only takes 10 minutes to set up.
EVDO Forums is a great resource for more information about this and other products.
Now you can sit on the grass in the park (within 100' of your car/truck) and surf the 'net at broadband speeds.