iCloud.com is live right now which means you can now access iCloud by logging in with your Apple ID account. Or well sorta. You're still going to need Lion 10.7.2 to get all the good stuff but you can poke your head in to check it out right now.
Once you punch in your credentials, you can see the framework of iCloud take shape: your mail, contacts, calendar and iWork are all backed up to Apple's cloud and the very useful but familiar by now Find My iPhone feature is packed in too. Cool! But without Mac OS X Lion 10.7.2 and iOS 5 out yet, you won't get to take full advantage of iCloud -- you're still missing Photostream, iCal syncing, account merging, etc. For now, it's just like a really pretty splash page that'll get infinitely more useful once you get loaded up with the proper software. But feel free to poke away! [iCloud]