Windows Phone 8 Developer Preview v8.0000.10 has appeared online, but it can only run on Windows 8 Release Preview.
Windows Phone 8 SDK Already Leaked : Read more
Windows Phone 8 SDK Already Leaked : Read more
Supposedly this leaked SDK, Build 8.0000.10, is the real deal, and requires Windows 8 Release Preview 64-bit to use, backing up previous rumors that the SDK would be a Windows 8-only release (both 32 and 64-bit).