With the just-launched iCloud, Apple isn't rewarding long-time Mac users with backwards compatibility. You'll need the latest Mac OS X 10.7.2, released just last Wednesday in order to automatically share content with your other devices. Mac users also need the latest versions iPhoto'11 (version 9.2) or Aperture 3.2, also just release last week.

Perhaps you believe that Apple software released before last Wednesday is quaint. Could be, but Windows users accessing iCloud can run anything back to the Windows Vista - which was released two years ago this week. iCloud supports Outlook 2007, released in January 2007.

So what gives? Wouldn't it be easier for Apple to at least support Snow Leopard and iCal 4 than it is for it to support Vista and 4-year-old Outlook?

Probably, but there are still more people running Windows Vista than all versions of Mac OS X combined. (See "Vista beats Mac but-Lion ties Linux.") Too many users to ignore.

In order to get Windows compatibility, Apple requires PC users to add software to Vista and Windows 7. This is the iCloud Control Panel. Once installed, Windows can use iCloud's Photo Stream and have contacts and calendars stored in iCloud and pushed to Outlook, iOS 5 devices, and Lion 10.7.2. Apple could have written a similar plug-in for Snow Leopard, but, well, it didn't have to.

For the troglodytes among us still using something earlier than Mac OS X 10.7.2 - say, Snow Leopard or Leopard - there are some iCloud features you can access. Basically, you don't get iCloud's push features, but you can use a web browser to check iCloud-based email, calendars, and contacts. You can't set up iCloud with these antique Mac OS X builds, you could set it up with your iOS 5 iPhone, iPad, or iPad touch.

If you're moving from Mobile Me to iCloud, you'll have to change your email settings, which Apple describes here.

Still, if you're running the world's most popular operating system, you're out of luck. iCloud doesn't support Windows XP.