Taking your cell phone to other countries can be a recipe for disaster if you don't take the right steps ahead of time. One AT&T customer learned the hard way that data roaming charges are no joke.A large number of us probably take unlimited data plans for granted. I sure do. I typically use between 1.7 and 2.1 GB of data on my cell phone every month, and often twice that with my wireless modem. When you're being charged by the kilobyte, however, using 1 GB or more of data while roaming overseas will add up to some pretty expensive bills.

In November 2008, a man was sitting on a cruise ship in the Port of Miami waiting for the boat to depart. In order to kill the time, he fired up his AT&T wireless modem, connected to his TV via Slingbox and enjoyed the Chicago Bears versus Detroit Lions football game. When he returned home, he was shocked to receive a bill for $28,067.31 in international data roaming charges.

How did this happen?

Despite being within range of AT&T's own network, his data card picked up the ship's cell phone network instead. Even though the cruise ship never left port, it is considered an international location (it connects to land-based networks via satellite), and incurred data roaming charges.

According to the Chicago Sun-Times, AT&T's system tried to warn the man by sending SMS messages to his wireless modem, but wireless modems have no way of displaying such warnings on a PC screen. AT&T charges 2 cents per kilobyte for data roaming out of the United States.

At first, AT&T offered to knock the bill down to $6,000, which is still an outlandish sum, given the ship's wireless network should have been off (as is customary when in port). Eventually, the Sun-Times helped argue the man's case and had the bill knocked down to $290.

In my recent trip to Spain, I made sure to turn data roaming on my iPhone off completely, along with its e-mail retrieval systems. Instead, I bought a local SIM card and put 20 euros on it for data, calls, and messaging. That got me through the week just fine. I'll take 20 euros over $28,000 any day.