Pre-orders and previews begin April 10, launches April 24.
Apple Adds iCloud Safari Tab Syncing to OS X Mountain Lion Preview 2
And although I couldn’t find this feature in the version I tested, Apple says that Mountain Lion will bring the ability to sync Safari tabs to iCloud, so your open browser tabs can sync between Macs.The features seems to be now implemented in the Developer Preview 2 that was released tonight. We received this screenshot which shows the feature button in Safari:
The feature is believed to sync open tabs between Macs and iOS devices, providing you with a relatively seamless browsing experience across devices.
Update: Here's an image of it working across two Macs.