Not in DSLR sales. If I remember correctly from the last sales numbers I saw, Nikon & Canon combined are around 85% of sales. That's a small slice of pie left for everyone else.There seems to be a trend in the press to count "interchangeable lens cameras" as a group and work units out as a market share. No surprise that shows Sony/Olympus/Panasonic growing at the expense of Canon/Nikon. The big question is therefore are "mirrorless" cameras going to kill DSLRs, and where things go from there. I wouldn't expect any quick change whatever happens.
If you're talking P&S and bridge cameras, that would be much closer to the truth; there are a lot of players in that market though.
Until there is fast and accurate CD AF, there's no way mirrorless will kill DSLRs. I could see it taking a significant bite out of the low-end market though (granted, that's where most of the sales are). And if someone does come out with a large sensor (APS-C or larger) mirrorless, with really good lens support, and AF at least as good as low end DSLRs; I could see it being absolutely devastating to the low-mid end DSLR market. Maybe even as second bodies for pros with less demanding AF needs (wedding/event).