Returning to the specifics of the opening post, rather than the more global issues which we are already discussing here [webmasterworld.com]:
I would not expect this just from the introduction of global search - one change that might push you down a couple of places, yes - but a day after day drop points to some other issue with your rankings.
The timing is probably just a coincidence. I suggest you start studying the SERPs where you are falling off and see what patterns jump out at you.
In order to implement Universal Search, Google had to find a way to put the relevance scoring for each specialized search on the same kind of scale so they could be integrated. From what I can see, the new measures were mostly in the area of making sure that too much news and video did not flood the results. A URL may have been pushed down a notch or two, but there's no ongoing lower-lower-lower thing happening to the previously well ranked URLs.