Microsoft computer accessories and Windows mobile are both struggling.

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@Mrstickball: I believe Xoj was talking about sales data, not estimates based on web browsing on selected sites.

Firstly, Symbian was supposed to be the standard platform for mobile devices, so the same reasoning you have for Android, goes to Symbian. What you gave as a strength for Android, concerning the carriers, is the weakness Nokia had in US market.

Having "only one iPhone" isn't excactly the case with iPhone for two reasons:
1. There's more than one model already.
2. Each new iPhone seems to have more powerful hardware than the earlier model. Which makes it no different than the phones running Symbian.

Every downside you can think with Symbian goes with Android as well.

If you don't want to program your games directly to hardware, you can use flash or Java instead, which makes differences in hardware less visible for the developer.

The mobile phone gaming have existed a long time, where you could download the game from the carrier or directly from the publisher. N-Gage wasn't made because Nokia inventing mobile phone gaming with it, but because people were playing games with their mobile phones. And the cheapo-phones do run 2D games.
The centralised marketplace is a new feature, but we are going to have marketplaces by manufacturer, instead of marketplaces by OS, which you were hinting with Android, which leaves iPhone OS as the only OS with centralised marketplace.

With all this, i'm not saying that mobile phone gaming wouldn't have its place or it would cease to exist, just that the current growth rate isn't going to continue and it isn't going to end devices like DS or PSP.
The biggest reasons for growth in mobile phone gaming have been the phones capable of playing games becoming more affordable, new entrants in the market and developers waking for mobile phones. But the developer situation isn't any different from the rest of the gaming market, where's growth, there's developers after easy money with their "LOL, casual games" (cheap shovelware) just before getting burned after someone releasing a killer-app, such as a fart simulator. Then the developers choose a new platform to molest after seeing some growth there.

The situation between handheld games devices and smartphones is similar to consoles and PC's. There's a lot bigger audience playing browser based flash games than there is playing games on a videogame console. PC's can even offer you comparable of better gaming experience than a console, but still what were looking here, at this very site, is, how the consoles are doing software- and hardwarewise on the market against each other and the sales are pretty good.