The process seems straightforward, you choose a number you'd like and it is added to your current mobile phone. The line has its own separate voicemail, and allows you to screen your calls while keeping your other phone number private.
The cost? $4.95 per month, plus an additional 2.7 cents per minute rate. You will also need a Blackberry, Windows mobile, or Java-enabled phone in order to run the software.
For more details, check out the Mobile Line 2 site.