@AndyC: If you want to go cross platform, build a website, why tie yourself to a single platform?
But apps that run on iOS don't run on Android or vice versa. There are 7 billion people in this world. Knowing that I can't possibly reach all 7 billion people is nothing that I'm too concerned about. You've got to pick your niche when it comes to software, and if the niche is defined by not only the domain only but also the platform, then so be it.
What Microsoft should have done is double their development team to focus on their desktop heritage, and the new unproven tablet and phone. that way they would keep their core business happy, whilst taking the gamble on tablets and phones.
I don't know why doubling their resources is necessary. They obviously devoted considerable resources to WinRT and tooling for Metro-style apps while neglecting WPF, but Silverlight recently received a refresh with its new out-of-browser capabilities (i.e. child windows, etc.). It looks like Silverlight is intended to replace WPF. Everybody is asking "Is Silverlight dead?" when the question to be asked should be "Is WPF dead?"