Well, this depends on their target audience.
If you have C/C++ code, porting it to ARM should be a huge deal. Yes there will be some differences, yes, there will be bugs - but in terms of effort its manageable. And more importantly, every single vendor has to do this effort, so Microsoft doesn't have to do anything.
Because Microsoft are saying this now, with no product that anyone can "buy" right now (or even soon), this probably means the audience for this news is *Developers* (the single intelligent word that Mr Balmer has uttered in the last 20 years, so good, he had to say it multiple times for it be considered a quote). They are now selling the ARM architecture to developers. If the developers buy this story, the applications will follow.
And of course, some developers will be more prepared than others. Don't expect an ARM version of Photoshop anytime soon, but an ARM version of Firefox is something that could be cranked out very easily.