it's certainly not going to be too long before ARM CPUs are perfectly suited for normal peoples' desktop needs.
x86 processors have a decade or two's worth of applications already built for them. Also, while these ARM processors are fast, they pale in comparison when doing any *actual* multitasking or heavy work. Looking past Photoshop and FCP, you should realize that ARM processors, at this point in time, are only running one or two small mobile applications at a time. Phones and tablets just now started to smoothly fill a 720p frame with an app and scroll through a list (well, Android, at least). Try having 14 tabs open with full desktop sites (one full facebook page brings my SGSII to a crawl), running Netflix in the background, and having a full Office-like experience open to type papers. None of that is beyond "normal peoples' needs," it just sounds like a college student writing a paper, looking for sources, and having background noise. None of that is possible on today's ARM chips.
I'm sure there will be an opening for a Chromebook-like market, but I can't imagine it destroying x86; especially not in 3 years.