From Apple's press release on iCloud:[N]ow there’s an added bonus way to burn up that [iCloud] free space: your PC. Apple wants me to use their Cloud-based storage on all of my devices not just Apple ones. Pretty darn sneaky, if you ask me. Smart too....To setup your PC to use Photo Stream, click the checkbox and then click Options to see the screen in Figure 2.
the storage required by Photo Stream doesn’t count towards this 5GB total
Took me about 10 seconds on Bing to find this. I guess technology journalism is hard.