Alongside Windows 8, smaller developers will be able to take advantage of the basic mobile package for free. Previously it had a licence cost of $800. It extends the Unity Free model which saw desk-bound browsers having access to the technology.
It's not a free ticket to use all of Unity for ever - there are some revenue and team size restrictions on Unity Free which would mean that bigger dev houses will need to look at Unity Pro ($1500) and the add-on Dev Team tools ($500), but for those bedroom coders and small indie houses starting out, this is a fascinating offer.
Unity has already seen some big name titles make the jump to Windows Phone (the Temple Run titles spring to mind), and this offer should give more developers the chance to use the engine to make some fantastic visual applications. I can't wait to see what people come up with.