I just hung my Nokia Lumia 800 to Zune to just some photos to sync and tadaaaa! There is an update ready!
Indeed, there is no one ready, but the whole process on Windows Phone 7.8 ending up takes about four updates all at once installed. If your device is already completely up to date, you will get first Nokia Update, then update to 7.10.8773 7.10.8779.8, then proceed with the update to 7.10.8783, which prepares the device for the final update to 7.10. 8858.136, to be exact. When the process is complete, you will in any case be provided with the opportunity to reduce your Tiles and there are a large number of changes. The splash screen is the same as that of the Windows Phone 8 devices and the Xbox Tile is now fully Windows Phone 8 style!
For those who can no longer wait, hang your Lumia 800 (or 900) to Zune and see if the update is available. It may be that branded devices have to wait, but hopefully also in the coming weeks the new version presented.