It really depends on what you want to use it for. If you are trying to separate friends from business there are a ton of programs that will do that.
If you want a second line for data, or more minutes, or to have business paid account, then the below will not help.
There is a ton of programs out there that make two numbers one phone. By far the best is google voice, that I have used so far. it is still in alpha, and I do not know when they will accept new people.
The concept is simple. Google will give you a new number. The number will ring through to any phone number that you want.
So let us say that you have (555)555-5555 on your blackberry, google will give you a number like (555) 555-5556. The new number will ring through to your phone.
The google number allows you to send any phone call to any number you want. For example, you could link your father in law to a local tavern. Or send your boss to your hotel in Israel (have not tried this, outside of the usa).
This will give you a second line, using only one line.