As we can see in the image above a much larger bead/lip defines the edge of this strip on these wheels, now both @F1Talks and many of the folks over on F1Technical.net's forum are concluding that this is indeed an aperture in the surface of the wheel (as indeed did I at first glance, until I looked deeper at the past design of the Mercedes wheel). Now going from what the eye see's (or perhaps wants to see) that could be one explanation with an inner structure of the wheel allowing airflow to permeate the inner structure of the wheel thus cooling the wheel and in turn the tyre. It's a fanciful notion perhaps but could a double skin within the wheel not only be strong enough to cope with the forces at hand but also allow air to transfer from one chamber to the other dissipating the heat?
Personally I don't think so I simply believe they have learnt from their experiences so far this season by coating the inner face of the wheel black and adapted it for use with the 2012 construction. Afterall we know that Mercedes decided to drop their 'Coanda' exhaust after Austin last year as they were over heating the shoulders of the tyres. Of course they have developed their 'Coanda' exhaust hugely for use on the W04 but it could still be a concern with a change back to 2012 specification tyres.
Another lesson in perception perhaps with the black strip simply being a coating to the wheel but it is still something that Mercedes are trying in order to reduce the temperatures and therefore degradation this season.