I've been abbreviating it to W10M for about half a year now. IMHO its one of few options that makes sense. This is why:
a)
It won't be long until this mobile SKU of the W10 OS (that's how MS wants us to think about it) will also be powering Windows tablets (like the Surface Mini). It makes no sense to call it a "phone" OS, when some of the devices this OS powers can't even make phone calls. This OS is for any smallish battery powered mobile device. That's why MS had to divorce itself from the phone specific name in the first place! It makes no sense to go against that.
b)
At WCentral we currently have a forum called "Windows 10 for Phones", but that is a stop gap measure. It works for now because this SKU is not yet available on anything but phones, but this will change at some point. Joe Belfiore called it "Windows 10 for phones and small tablets". That's what it actually is. It's just too long for practical use, but dropping the ending makes it incorrect.
c)
What remains of Nokia goes by the name of Microsoft Mobile, and the team at MS that develops this OS is the Mobile OS team... Mobile!... hint! hint!
I think "Windows 10 for small mobile devices" is an accurate description. It's also easily shortened to W10M. It's what I'll be using.