My thoughts on mobile OS's.

There are 3 outstanding competitors; iOS (v4+), Android (v2.1+) and MeeGo.

Obviously iOS is cross-compatible (iPhone, 3G, 3GS, 4, iTouch, iTouch3, iPad) but not across different standards (ARM, x86, RISC) and only supports hardware manufactured by Apple, which is a big disadvantage and I don't support it.

Android is everything I like; its touch-based OS for underpowered devices, cross-compatible and multi-platform (x86 & ARM). It is average complexity (has its limits), has a great UI, already available widespread and very popular. It is java-based so Apps are relatively easy to write. All the big plus'es, but it is much reliant on the VM so Apps run "slower" and basically to make a port you need to start from scratch.

MeeGo is "smarter" than Android in some aspects. It is exactly like the Android platform (touch OS, great UI, cross-compatable, multiplatform) with two exceptions. This exception is in its solution to cross-compatability. Instead of java/dalvik, it uses Qt, making it possible to keep its cross-compatability even across platforms. Qt allows the software one step closer to the hardware, so it can run "faster" than java-based software. In addition, Qt makes it possible to port existing Linux apps (and there's many) to the MeeGo base. The second exception is that MeeGo has strict rules on UI, you may apply skins/themes but each version will require the same UX. This is a good thing, you can customize things but there is a limit, so if you used MeeGo on X device you know how to use Y device. And this potentially prevents fragmentation. There will be several MeeGo versions, each like each other (kernel etc) but offer different UX (eg Smartphone vs Netbook) for obvious reasons. And all Apps will work on all versions.

So MeeGo is "potentially" the best OS available out there (truly, even against Win7) but it has many ways to go before it can compete with Android, and an eternity longer if it were to compete against Microsoft in the high-powered (laptops, desktops) devices sector.
With MeeGo its possible, you may only ever need to know two OS your entire life: a high-performance OS (eg Windows7) for high-powered devices and MeeGo for all other computers.