The commonly accepted date is 16 September. Xcode, which is the development tools piece of Panther, is shown to be shipping 15 Spetember on Apple's Web site. There's also been some news reports that indicate mid-September as the "expected" ship date; Apple has not said anything contrary to this. Some will point out that Jobs said something like "before the end of the year" at WWDC, but it seems to me that Apple has used this type of statement for virtually all forthcoming products (iTMS for Windows and Xserver Panther, for example). I think this is just Apple-speak for "we're not going to tell you yet".
There's another thread a while back to speculated about this topic.