Microsoft has not yet specified a release date for Windows Phone 8. Hardware partners like Nokia already showed their Windows Phone 8 devices, but the OS is not ready.
Nokia unveiled this week the Lumia 920 and Lumia 820 Windows Phone 8 devices and they look very promising. The event lacked pricing and release dates. According to Arstechnica the reason for this is that Windows Phone 8 is not ready.
At the Nokia event the phones have not been available for the press to play around. Windows Phone developers still do not have a developer SDK for WP8. Rumors saw the release of the Windows Phone 8 devices at the end of October. This seems under this circumstances not likely.
The reasons for the delay are likely the change to the NT Kernel and APIs.