There isn't a unique PIN, per se (although there is an ESN).
You can't send messages directly to the device (like you can with BlackBerry).
There are several IM-type apps that work on Android, which have similar functionality as BBM. Those apps include Kik, LiveProfile, PingChat, WhatsApp, and the standard IM apps (Google Talk, AIM, YM, MSN...).
Most of the non-standard IM apps (Kik, LiveProfile, PingChat...) work sort of like BBM. They usually include the S/D/R confirmations. Some let you create your own profile name, others assign a PIN to you. If you change devices, you get to keep the same PIN, since it is not tied to the device (like PINs are with BlackBerry).