Attention users of BES12: Enterprise Instant Messenger is back. According to the BlackBerry devblog, EIM was no included with BES12 because the required component, the BlackBerry Collaboration Service for it, was missing. With the BES update to 12.1, the missing piece of the puzzle has returned.
The BlackBerry Enterprise IM client for Cisco can be found on BlackBerry World here:
For more general info on EIM, check out this business blog entry: