Does it mean less demand for iOS and Android developers?

"Microsoft has purchased Xamarin, a startup that helps developers write apps once and have them work on any smartphone or operating system, according to anofficial blog entry."

I think Microsoft acquired them to get more iPhone and Android apps on Windows Mobile. 

Yes, this is so, but if this technology becomes widely available, would it mean a company that wants to release on multiple platforms is not going to need developers for all those multiple platforms?

Yes, that's true. There will be less need to specialize.