There is not denying the benefits of purchasing an unlocked cell phone with your next cellular purchase.  However, it is important to make sure that the unlocked phone you buy will work on your network.  There are two types of technology used in the US on cellular networks - CDMA and GSM.

As I explained in my post about Activating a Used Cell Phone, the two technologies are mutually exclusive. A phone designed to work on CDMA technology will not work on GSM technology and vice-versa.

However, a phone that is designed to work on Verizon's network that is unlocked will then be able to work on any other network that uses the same technology that Verizon does. Since different providers offer different models of cell phones, this opens up your options for your next phone, so long as you shop for an unlocked phone.

The important thing to know, then, is what type of technology your cellular provider operates on. Although you may find a great deal on an unlocked refurbished AT&T Blackberry, if your provider does not operate on the same technology, you will not be able to use the phone.

Below I've noted the "big" cellular providers and they type of technology they operate on. A more thorough list can be found here.

AT&T - GSM
Cricket - CDMA
Sprint/Nextel - CDMA
T-Mobile - GSM
U.S. Cellular - CDMA
Verizon - CDMA

So, what you should gather from that list is that you can purchase an unlocked refurbished Blackberry for Sprint and activate that same phone on U.S. Cellular's network.

This is helpful information to have on hand when you're shopping for your next phone as you will not have to limit yourself to phones designed specifically for your network, which will give you more options and probably will save you some money.