The following Gartner chart, via a Between the Lines piece on whether the mobile OS wars would follow PC history, shows the forecast:
It's only an educated guess, and as such is quite possibly all wet. However, if it's even close to what will happen, it suggests that over the long haul, platform will win out, which makes sense. Economics and availability end up winning out. But unlike the PC industry, there may not be a dominant platform. Gartner suggests three platforms by 2015, with some niche players. One split would apparently be in smartphone versus "normal" handset. Many people simply don't need and probably won't want (and won't pay for) more than a phone with some features like a camera.
Although Android could clearly offer that, Symbian will likely stay around because manufacturers avoid the design learning curve and still give most customers what they wanted. The question will come down to Apple versus RIM, and although the current trend has Apple coming out on top, it may be that RIM's better connection to enterprises -- not just users, but IT people who have to ultimately approve devices -- might keep it as a fourth player.
[UPDATE: A reader had emailed me the other day asking why I didn't look at the potential of WebOS. Via GigaOM Palm Pre sales are in serious decline, according to one analyst. Some on the thread at that post claim that the analyst is biased against Palm, and another is positive about the company's outlook. My thought: If Palm's products were doing so well competitively, then why the need for a $99 price?)