According to the latest information the GT 300 has not even taped out. So if the GT300 tapes out in the next few weeks we would still seem a very unrealistic time line for testing, processing and then production by the AIBs.
Charlie Demerjian, at the Inq has the whole sordid story on how things work in the production world. And if his calculations are right we won't be seeing the GT 300 until very late December perhaps early January and that is only if everything goes perfect. Realistically it is more like June-July 2010
Read more here.
That puts testing of A0 silicon starting August 1, and that will take about two weeks. If there are no bugs, or very minor ones, production can start on August 15, and that takes 10-12 weeks. Ten weeks from wafers in brings you to Nov 1, which is slightly after the claimed Oct 15 date.
Assuming the AIBs need a few days to build the boards and ship them, you likely won't see them on the shelves until mid-November at the earliest. All of this has the assumption that thing go perfectly, something that we have only heard of once in A0 GPU silicon.
If they have to do only one respin, that puts Nvidia out ten more weeks, eight for silicon, and two for testing. Nov 1 now becomes Jan 15. Since Nvidia is 'teh awsum' at making new parts of late, a second respin isn't all that much of a stretch, so it could be out around this time next year.