At an event in London today, Ballmer said Microsoft (MSFT) will debut its own “cloud operating system” at its Professional Developer Conference at the end of this month. “We need a new operating system designed for the cloud and we will introduce one in about four weeks, we’ll even have a name to give you by then,” Ballmer said. “But let’s just call it for the purposes of today ‘Windows Cloud.’ Just like Windows Server looked a lot like Windows but with new properties, new characteristics and new features, so will Windows Cloud look a lot like Windows Server.”

Ballmer offered few details beyond that, saying only that it will enable “light editing” of Office documents. Presumably, that means Microsoft still doesn’t see products like Office moving entirely off desktop PCs and onto the Internet any time soon. I imagine we’ll be hearing quite a bit about Software plus Services later this month.