It's built into the normal iPhone texting application and turned on by default. When my Mom texts another Apple user, iMessage will automatically route her message over the Internet. She doesn't have to approve this, and honestly, probably won't even know the difference.
Do you know if messages are encrypted from device-to-device on BBM or if they are just encrypted to the hub like I message seems to be
The same is not true for BBM messages sent via the BlackBerry Enterprise Server (BES). Those are also encrypted, but using a key possessed only by your company's IT department. That means that if someone wants to read you messages, they have to subpoena your company. RIM can't help them, at all.
I'm not sure what happens when a BlackBerry connected via BES sends a BBM to a person using BIS, or even another BES network. Either decryption and re-encryption occur, or it reverts back to using RIM's private key. But it would be safe to assume that BBM messages sent within your own company's BES network are safe and secure.