Nvidia possibly wants to outpace AMD and introduce the GT300 on a pre launch demonstration in September.
Nvidia GT300 pre launch event in September?
While the RV870 is apparently ready to go, Nvidia hasn't revealed much details of the GT300 yet. But this could change soon, since it seems like the Geforce creators don't want AMD to get all the fame for the next gen GPUs. According to brightsideofnews.com Nvidia will introduce the GT300 in late September to selected people (graphics card manufacturers and press).
Given that fact that this presentation is most likely to be a pre release event it will take some time after before the first GT300 devices will hit the retail markets.