Unfortunately you used the key phrase "equivalent quality" in your question, so my answer is "no".
I own the 24-70/2.8 Nikkor and my opinion is that it is without any question one of the finest mid range zooms made (by anyone) and I'm quite particular about image quality. Both the objective and subjective reviewers who have reviewed it so far have given it very high praise.
This does not mean you can't "get by" with a cheaper 3rd party alternative, but it's not going to be the same.
At the end of the day it comes down to your budget and philosophy. My approach to lens purchases is that I'd rather buy the best glass I can afford, one time, instead of getting into a cycle of buy/sell/trade.
Excellent glass is a good investment in that you won't generally be replacing it as often as the nice shiny new DSLR temptresses, er, bodies that come out every year or two
However, the decision is a personal one. The lens is rather rare at the moment, but perhaps you can borrow one from a friend or rent one once they become more available and make your own decision. I'd make the comparisons on a D300 body if I could - the higher resolution body will both stress and show off the lens quality more than the D70 would.
Hope this helps.
-mike
