Should you create a wave, you can now grant either full access or read-only access to participants you invite to join. You can make the change by clicking on the user's avatar at the top of the wave panel and using the drop-down menu to adjust his read/write status.
Thankfully read-only privileges extend to groups, which means you can now finally make a public wave read-only as well. This should help eliminate the chaos that public waves tend to create, and make them all the more useful to businesses and power users looking to share their always-updating waves, without compromising wave quality.
In addition to the read-only update, Google Wave users with full access to a wave can now also restore that wave to a previous version using playback mode. The new feature is akin to reverting to a previous saved version -- a feature in most word processing programs -- but with the additional granularity of being able to pick an exact moment in time over the coarse of the wave's entire history.
According to Google's announcement of the new features, you can also soon expect to see a reply-only access level, which will let those users add blips but not edit them, and Google Wave interface tweaks to simplify bulk permission changes.