When I am not around free WiFi I use my Kyocera KPC650 Aircard with Verizon high speed internet cell service. Has native powerbook support. This is a very nice mobile internet solution with good coverage over most of the country. This card requires a PC slot, so it does affect which type of Mac you need. They should be coming out some smaller aircards that fit this configuration before too long.
Also, supposedly Kyocera makes some type of standalone router that accepts an aircard and then provides a WiFi signal. People say this works.
The cell phones that connect to a Mac via a wire aren't supported very well (and in some cases are prevented from working) and are sloooooow.
The newer powerbooks have a smaller card slot than the standard and supposedly some aircard type solution is coming out for them.
The site www.evdoinfo.com
has some information about support for internet cell aircards for Macs.