This is in addition to Chris Wuks post...

I have a i90c that uses the nextel network (with the direct connect, wireless web, etc.). My phone also uses a sim card. The sim card saves all of your preferences, phone book, web favorites, network information, etc. You can move it from phone to phone. So... if I wanted to upgrade my phone, I don't have to delete all of the settings in my old phone and enter it all in my new phone. Simply pull the sim card out and put it in my new phone. Keeps the direct connect settings, phone number, phone settings which will be applied if the features are available on the new phone, and phone book will all cary over.

Pretty good idea. I am pretty rough on cell phones and I can go through 2 or 3 a year.
I was getting tired of having to enter in 50+ numbers and settings everytime I upgraded or got a new one.

Here is some more info about sim cards. Here is another good one.
Since this is a security site... here are some links to sim security concerns...
http://news.zdnet.co.uk/story/0,,t269-s2109824,00.html
http://www-5.ibm.com/services/za/suc...dy_120257.html
http://www.theregister.co.uk/content/59/25216.html

There are several more out there...