Apple introduced iCloud earlier this year, and its announcement kicked off a renewed interest in the iconic cloud logo. Articles espousing the icon's Golden Ratio-based design and Apple's attention to detail followed shortly. Though Apple undoubtedly chose this icon for its precision design, it's likely that Apple's design team can't take full credit for the look.
A post by Scott Hanselman points out the cloud icon may have its roots in the Pictos 1 icon collection. Though it's been modified some, the basic circular cloud design has been widely used. It appears on Microsoft's ASP.net website and in many iOS weather applications. A very similar cloud design even showed up on a BBC weather segment almost thirty years ago. You can check out his post for even more examples of this cloud configuration.