Windows on ARM has been talked about before - in fact, I firmly believe that Windows NT has been running on ARM inside Microsoft HQ in Redmond for years. I'm not sure how much of it is left, but Windows NT was originally designed to be highly portable, so much so in fact that the first versions of NT were coded on boards designed internally at Microsoft (Dazzle (Intel i860) and Jazz (MIPS)). Earlier versions of NT ran on x86, PowerPC, MIPS, Clipper, and Alpha, and a port to SPARC was under way at Intergraph.
So, NT's roots lie in portability, but how much of those roots are left remains to be seen. However, even if Windows NT can run just fine on ARM, the bigger problem is of course the massive body of applications that do not. Then again, with ARM clearly being an embedded thing - in this case, tablets in particular - the usual Windows applications might be useless anyway.
Anywho, what's Bloomberg reporting? "Microsoft, the world's largest software maker, will announce a version of its Windows operating system that runs on ARM Holdings Plc technology for the first time, said two people familiar with Microsoft's plans," they write.
That's it, basically. Those two people might as well be talking about Windows Mobile CE Pocket PC Embedded Compact (or whatever it's called these days). They're also not saying anything about how close to Windows NT-regular it would be, and how, and if, you would run regular applications on it.