From the Nokia Power User article, following a business contact sending in an official letter:
A leaked Nokia’s letter to its existing Devices and Services business suppliers base reveals two interesting things. First revelation is about renaming of Nokia Oyj to Microsoft Mobile Oy, which will be a wholly-owned Microsoft subsidiary and will be the mobile devices arm of Microsoft. Good thing for suppliers is that current terms and conditions that they have with the Devices and Services business will not change post deal-closure.
"...Please note that upon the close of the transaction between Microsoft and Nokia, the name of Nokia Corporation/Nokia Oyj will change to Microsoft Mobile Oy. Microsoft Mobile Oy is the legal entity name that should be used for VAT IDs and for the issuance of invoices."
Of course, there's no guarantee that a company name change in one country will necessarily apply across the rest of the world, but Microsoft Mobile does seem like a logical choice.
I (personally) commented several years ago on AAWP, fuelled with a little suspicion and paranoia, no doubt, that the very name 'Lumia' was partly chosen as a 'brand' which would be portable as and when Nokia sold up to Microsoft, and my guess is that this is indeed what will be happening. Consumers will still see the popular 'Lumia' brand, front and centre, on shop shelves and the only change will be that the manufacturer name (presumably in a smaller font?!) will alter to reflect the new ownership.
We'll comment more fully once something officially gets announced in this regard, of course.