I wonder what features will distinguish the Nikon as "pro"?
I guess Nikon can steal the Sony sensor from the D7000 into a compact, mirrorless body along with the stuff that makes Nikon "pro" - lots of buttons and switches to avoid menu diving, a hotshoe that supports Nikon's CLS strobes, etc...
But just like Sony, the challenge with this approach is a lens lineup that takes advantage of the small size. I can't, for example, imagine taking the wonderful 12-24 F2.8 and 24-70 F2.8 and attaching them to a small, mirrorless body. The lenses are the bulk of the package, so any savings in body size is minimal.
Maybe, just maybe, they'll do something crazy, like an FX sensor in a mirrorless body and a trio of primes - 24, 50, 70 - all fast (F2.0 and faster), all small, all awesome. That might be something I could get excited about!
The way I see it, mirrorless (specifically, large-sensor) systems will make SLRs obsolete. Eventually, the sensor, processor, and screens will get to such high quality and ability that the only reason the EVF will look different than the OVF will be because it's supposed to (e.g., made brighter to aid in focusing), not because it has to. Some day, there won't be new SLRs, just new mirrorless cameras with full function (AF, etc.) adapters for the legacy SLR lenses. The question for manufacturers is how long do they wait before investing in the new technology. Right now, there are still differences that make mirrorless cameras not necessarily an upgrade, but rather a side-grade. Sales of SLRs will stay strong until that gap closes. These manufacturers can delay investment due to the strength of their existing lines, letting other companies figure out the do's and don't's for them, without the risk of losing too much ground. However, they will hit the critical point at some point. Nikon seems to have taken the plunge sooner than Canon, but they will follow suit eventually.
What :43: has going for it is another factor that nobody has copied: the smaller sensor. With that smaller sensor, the whole system - lenses and bodies - can be much smaller. With larger sensors, competitors may get better IQ, low light performance, etc., but the size tradeoff (see Sony lenses, for example) means that there isn't a clear winner. They have two different primary markets and each suffers compromises in crossing over to the other. Until someone else comes out with a - for lack of a better term - compact mirrorless system, :43: will continue to remain strong.