I don't think it's really a matter of desktops becoming like mobile devices. I think it's "desktops as we know them will be going away (except in specific use cases)".
My wife's Macbook has seen better days. When it finally dies, we're going to replace it with an iPad, a bluetooth or docked keyboard and a camera connection kit. It does everything she needs. I imagine this will be the same when my kids get old enough.
And why is ARM a bad choice? Have you heard about performance complaints from iPad users? Have you seen Rage HD on iOS yet?