The Twitter account of @Pontifex was designed to be used by Popes. Pope Benedict XVI is abdicating on February 28 – meaning that he will no longer be Pope after that date.
Once Pope Benedict XVI formally abdicates, he (among many other more important matters) will forfeit the right to use the Papal Twitter account. As no one would be Pope until the conclusion of the conclave to elect a new Pope, it would make absolutely no sense for the Papal account to be active after February 28.
Pope Benedict XVI is leaving to his eventual successor the decision of whether or not to use the Papal Twitter account. And it would be very foolish, at least from a PR standpoint, for the Pope to establish an Twitter account, only to have it permanently shut down a few months later. (Admittedly, the leaders of the Church are not particularly prudent when it comes to public relations.)
So is Benedict XVI leaving Twitter? Probably. But is the Papal Twitter account gone forever? Not likely.