From the way things are going it looks as though it won't be hitting the market until ATi's R8500XT becomes available. Until then the GF3 Ti200 and GF4 MX460 are managing to compete reasonably well in the market against ATi's current entrant into the price range that the Ti4200 would occupy.
I wouldnt expect to see it for another 3 weeks at minimum, and wouldnt be terribly surprised if that stretched to about 2 months before we see them hit retail availability.