i think that this also has to do with enterprise customers abandoning the blackberry system in droves. With more and more companies moving to the Individually Liable models for phones, they don't want to pay for a BES server+BES admin staff.
One of carrier partners is selling android devices a direct replacement for blackberry devices and being hugely successful. WinPho isn't a viable option yet (and 6.5 WinMo is a burning platform), symbian's a joke, they can't GIVE Palms away and the Jesus....err...iPhone simply isn't secure, or even a very good phone, for that matter.

Android has become the only really viable choice for mobile users, in the enterprise space, at least. When one carrier sells 10k android devices to a company every week, it's adds up quickly.