Tim Cook is currently demoing iCloud's integration in Mountain Lion onstage at the WWDC Keynote in San Francisco. iCloud, Apple's cloud sync service, has been a big hit on iOS already, with 125 million iCloud users registered and using the service currently.
On Mountain Lion, iCloud will come in the form of Document syncing, which Cook showed took only a few seconds to sync a document from his iPad to a MacBook Pro. Reminders, Messages, and Notes are all being synced across iCloud as well, and presumably will work just as smoothly as they work currently. Safari browsing history and bookmarks will also get synced, and even tab views can be synced between devices, so you can view tabs on your MacBook Pro, and then take those exact browser tabs over to your iPad or iPhone.
It all looks great. We should hear an official date for Mountain Lion soon, perhaps even later on today.