Interestingly the latest version of OpenSolaris (2008.11) includes a feature called “Time Slider”, which allows you to drag a slider to get to an older version of the file system state. I’m not sure if this leverages ZFS’s features or if it’s just hard-linked. Comments on the Javalobby site by Roman Strobl indicate:
Btw I believe what we have in OpenSolaris is better than time machine because a) you don’t need to use an external disk, b) the snapshots are immediate and don’t consume extra space other than differences from your current disk contents c) you don’t have to activate the backups, they happen automatically. So you also get more granular access to history. This is the first version of the feature so we plan to improve it in the next release.