FAQ for
Mission: Impossible III (2006)

According to Carnahan in an interview on joblo.com:

"Danny Gilroy... and I wrote a script about private military and going into Africa and assassinating this West African premier and throwing this country into chaos. Ken Branagh was gonna play this guy based on Timothy Spicer, who was an SAS colonel, British army. And he retires and hes a billionaire two years later, because he puts his private military together and they go into Sierra Leone and they go, You got a problem with this rebel uprising? Will crush em. 15% of the copper mines, 10% of the diamond trade, etc. We were so far ahead of the curve, with private military... But we had it where it was geopolitical and really sharp. It would have been something else, dude."

http://www.joblo.com/interview-joe-carnahan

Apart from the possibility of Benji Dunn's 'Doomsday device', the Rabbit's Foot was a MacGuffin - a device used to move the plot along, similar to the briefcase in Pulp Fiction. A MacGuffin usually doesn't require much of an explanation, because what it specifically is has little bearing on the story. All the audience needs to know is that it's something very important or valuable, and that all the players in the story are trying desperately to get their hands on it.

It's a Nokia N92.