For those who don't know, an API is a kind of middle-man interface between to programs, so they can communicate with each other.
For instance,  a Facebook app for iPhone and the Facebook website communicate via Facebook API.

RIM announced during the last Devcon Asia they would open Blackberry Messenger API later this February.

Here is why I think it could be game changing:

Let's start with some facts and figures:

• The key features of BlackBerry services are push email, integration with enterprise services and instant messaging
• BBM offers features pretty similar to social media services (photo sharing, calendar, etc...)

• BBM is used by more than 33 millions users worldwide
• RIM, by now, ties with apple when it comes to the amount of sales (behind Nokia with ± 14 millions units end 2010)
• The brand is available from approximately 565 carriers and distribution partners in 175 countries

Now with the crystal ball.

A daring bet

Where will the new users register, and how?
Through BES (paid) only, that would mean they would allow a one way only communication, potentially discouraging potential buyers who would have opted for their products for the IM service.
Making it free would mean even more potential loss, and the possibility for the BBM protocol to fall into oblivion.

Both methods are risky. Why taking this risk?

RIM could be the new Yahoo!

If now yahoo claims to have around 260 millions users, we all know where they're at.
Their innovation strategy toward the public as a disaster, and they are resting on their main services awaiting an epiphany that might as well never come.

RIM, though,  is in a similar situation as the early Yahoo!, with a powerful e-mail service (even if only push) and a fast growing IM protocol, but with the enormous advantage of being already well implemented in the mobile market and enterprise culture.

So yes, RIM could very well be the new Yahoo!, leading IM and e-mail for the next decade.

RIM can take on Apple and Google, frontally.

RIM can go where Apple and Google can't.

On the mobile market, they tie with Apple and best Google, that's no secret already.
But when you think about it, they have an advantage on one decisive aspect: the social factor of their user base.
Google has been desperately trying to get social with the failed Wave and Buzz experiments, and Yahoo! as done... practically nothing about it.
RIM has integrated many social features in their messenger already, statuses, media sharing and commenting, message broadcasting, and, of course, groups.
That is to say, BBM user base is social-media ready by nature, and already has a platform.

What could they do with it?

At that point, everything is but speculations. But there are some possibilities:

Opening the API in an OAuth kind of way (single shared log-in for several services) would provide a centralized and portable ID to access multiple services. Data monitoring, from that point on, looks like a logical source of revenue/information.

Setting up a persistent social platform based on BBM features has a lot of potential.
With their e-mail client, RIM has shown its expertise as an content aggregator, and with such a user base the occasion is golden for them to become both a provider and a connector in the social media field, especially since they already have a whole ecosystem ready to sprout around it.

2011 promises to be very exciting.