Earlier this week, it was discovered that Microsoft is working on a 'Cloud Shell', which is meant to be a "lightweight Windows version for the modern computing world". Now, references to the Windows 10 Cloud SKU have appeared in the latest Windows 10 Insider Preview build, 15019.
First noticed by Petri, the reference is found in the Pkeyconfig.xrm-ms, which can be read by using a third-party app called Product Key Configuration Reader.
You'll notice from the image that there are three versions: Cloud OEM, Cloud Retail, and CloudN. Note that 'N' versions of Windows are nothing new; those are the ones that ship without certain bundled services in the European Union and South Korea. But a retail version of the SKU seems to imply that not only would this Windows 10 Cloud be available on purchased PCs, but you'd be able to buy it as well.
Of course, nothing is written in stone until Microsoft officially announces it, and sometimes, even after that. Windows 10 Cloud may never show up in the real world; keep in mind that at one point, there were references to Windows 10 RT as well.