Boy Genius Report has provided tentative evidence that both iPhone OS 3.1.3 and iPhone OS 4.0 are being used in some form of testing either inside or outside of Apple.
The only information provided is an image containing references to both iPhone OS 3.1.3 and iPhone OS 4.0 which for the sake of clarity can be easily spoofed by adjusting a browsers user agent string.
The current public version of iPhone OS is 3.1.2 was released back in early October making it very like that 3.1.3 is in the works and likely to be presented to developers in the near future. iPhone OS 4.0 would be a major OS release and going on history would see a release in June next year.