I'd say the key difference is that mainstream people actually download and use apps on the iPhone, much moreso than on other platforms.
The businness crowd on blackberries probably send quick emails more often than they use SMS or IM (particularly AIM)
That's why, for example, it's cheaper to use T-mobile's $6 unlimited web access with Gmail's mobile app (which apparently uses http(s)), than to use T-mobile's email client and an SMS package. That of course doesn't apply to a phone with a required data/text package like the Sidekick.