Apple doesn't make a laptop with built-in 3G. That's different from Windows laptops, where many have an option for built-in 3G.
Thus you have three options: tethering from a phone, a 3G dongle, or a mobile hotspot (like a MiFi). Which is going to be best will depend on how much you plan to use it and how often. Note that some 3G dongles may not support Mac OS X, a mobile hotspot has battery life issues, and tethering may drain your phone battery faster than you like.
In the USA, some companies like Verizon and Virgin Mobile have pay-as-you-go mobile broadband. This is attractive if you only need it occasionally. Scottsdale's information is out-of-date - Virgin Mobile changed their pricing plans several times in the last few months, and it's no longer the great deal it used to be.
Bottom line: in most cases, you pay dearly for the convenience - mobile broadband is generally much more expensive than home broadband, it's less reliable, and it's much slower.