the way you do it is if you have BB data plan you go to yer BB messenger and open it>press BB menu button>add a contact>scroll all the way to the top highlight "use once">press BB menu>request by pin>then you eneter in the persons BB pin
Yup, like the others have already said, its unique to only blackberry users and its free. So instead of using regular text messaging and taking away from whatever package/plan you have, you can text for free as much as you like to any other berry user. Its handy.
This would apply when using text msgs (SMS) which has separate charges, but it doesn't apply to AIM, MSN, GTalk, Yahoo IM clients which also uses data and not SMS.
Companies who provide BlackBerrys to employees can take advantage of free Instant Messaging without adding costly, unproductive services like texting (or requiring that everyone have the same IM provider).
Employes who may need to communicate about a work project can simply communicate to the other using the built-in BlackBerry IM. I think it's very useful and smart.