One of the things Whisper had was a method of encrypting SMS messages so they still fit in an SMS message, perhaps Twitter is hoping to take advantage of that?
This is why I think this is such a strange move on Whisper Systems part. Their products are all about privacy. Twitter wouldn't have a business model if they gave their users proper privacy.
Social media is said to play an increasing role in revolutions and social movements; there's clearly something to do. Perhaps even, let's be audacious, some money to make.
Note: I don't actually think RIM is quite dead yet.
What's needed in those cases isn't peer-to-peer encryption, but peer-to-service (and service-to-peer) encryption: tweets encrypted on the device, sent, and decrypted on Twitter's servers; timelines sent encrypted, and decrypted client-side.
Twitter still gets plaintext, but intermediaries can't trace/target pseudonymous users (or filter content).
This could be a real edge for Twitter in countries (China) where they're losing ground to monitored/censored clones (Weibo).
tl;dr: They're probably building Tor, not Skype/BBM.