It's a subtle change but an important one: Apple's published Windows system requirements for iCloud are a bit different than they are for MobileMe and the current version of iTunes. Per the company's press release:
Using iCloud with a PC requires Windows Vista or Windows 7; Outlook 2010 or 2007 is recommended for accessing contacts and calendars.
Emphasis mine. What's missing? Only the world's most popular operating system (still!), Windows XP. While iTunes 10.3 and MobileMe both include XP SP 2 in the support list, it looks like the next stop on the road to the cloud is a no-go for the older OS.
Considering that Apple's backwards-compatibility on iTunes 10.3 only goes back four years for its own operating system, it's kind of surprising that the ten-year-old XP has stayed on the short list for as long as it has. This shift, however, may mean that some PC users will search for alternatives to Apple's tools for iOS support (assuming that iTunes itself will move up to match the iCloud specification).
Are you running Windows XP on any machines you currently use with iTunes or MobileMe? If so, is the prospect of iCloud requiring Vista or Win7 keeping you up at night?