Is Windows 7 SP1 already being worked on?
No, that doesn't make a lot of sense. You be the judge. According to information from Winfuture.de and Wzor.net - the Russian site that always seems to have early information about leaked Windows 7 builds - Microsoft has already begun working on SP1.
Critique the screenshot yourself - the system properties screen is open and clearly shows "Service Pack 1." The watermark text, curiously, is partially obscured and also includes the text "Temporary Censorship." The build number is 7138, one up from the most recent leak. It certainly looks like the image has been monkeyed with. Then again, we did just have that leaked box art which many people thought looked doctored as well - and that turned out to be legit.
Publicity wise, it seems like a Vista-esque bad move to have an SP1 build in the works before Windows 7 hits RTM. It gives the impression that the release isn't as good as it should have been. Of course, it's also inevitable that service packs will appear at some point in the future, so Microsoft could very well be laying the initial groundwork now in a separate development branch.