Once the file-system snapshot plug-in is installed for yum, once any package transactions take place from there on out it will create Btrfs snapshots. Each of the Btrfs snapshots are then named yum_<time-stamp> and when the default sub-volume is mounted, they can be found as yum_<time-stamp> sub-directories of the root file-system. Unfortunately, that is where the Fedora 13 support for system rollbacks ends.
Planned to make for a polished system rollback experience is pushing support for choosing Btrfs snapshots to mount via GRUB and the GNOME Disk Utility. This way if something goes wrong during the boot process of the newly installed or upgraded yum packages, the user can simply reboot the system and then choose a different snapshot to boot from the GRUB2 boot-loader. Unfortunately, this did not make the cut for Fedora 13 so users will need to wait until at least Fedora 14.
Support within the GNOME Disk Utility / Palimpsest for Btrfs would allow picking a snapshot and then the next time the file-system is mounted, it would switch to that snapshot seamlessly. This also is not to be found in Fedora 13, but will hopefully be integrated upstream for GNOME 3.0 and then found in Fedora 14. There has also been talk of integrating similar capabilities into the Nautilus file-manager and even with slider support for rolling back the file-system (similar to the support found with the ZFS file-system on recent releases of OpenSolaris), but this too had not yet materialized.
While there isn't yet any graphical support for switching Btrfs snapshots, a different snapshot / sub-volume can be mounted using the subvol mount option, such as mount -o subvol=yum_20100414183435 /dev/sda1 /media/disk. New snapshots can also be created at any point using the btrfs ctl -s <snapshot-name> command. It is too bad though that a full-blown system rollback support feature could not be completed in time for Fedora 13, but that leaves something to look forward to for Fedora 14 later this year. Hopefully by the time this work is completed the Btrfs file-system will have matured a bit more, received more performance optimizations, and in a state to be more widely adopted.