They only did this for tablets and Android.
See, they know Android would flood the tablet space soon and unlike the mobile market, they don't want to be so late to the party with a solid solution.
Additionally, they don't want to have to build a windows mobile phone 7 for phones and then a upgraded/better OS for tablets. Instead they will use the already dominate Windows7.
Why? Well, ARM CPUs will already be used almost exclusively for tablets due to Android, QNX, and WebOS. So, to keep manufacturers costs down and get them to sell MS OS's, MS made a Windows OS that works with ARM.
Very smart and the best way to get a Windows tablet(s) to the market just as Android is really kicking off in that space. They don't want to be years late to the party as they were in phones. (yes I know Windows had phones previously, but they seriously were no where near the experience found in Android, iOS, and BB).
EDIT: as for the laptop/desktop market, this means nothing. They will remain x86.
