I read in a previous post here that it's not possible to revert from win8.1 back to win8 (or whatever OS was installed before). That's not true though: http://wind8apps.com/uninstall-windows-8-1/
Just to be clear, a system restore may or may not return you to your previous version of Windows 8. A system revert (refresh or reset) definitely will not. And neither is possible if you have installed the 8.1 preview over an earlier version of windows or if you install it from the iso file after formatting.
System restore depends on having all of the files and settings present in a restore point in the shadow copy. If multiple restore points have been set, the earlier restore points (and the files and settings they "protect") will be overwritten as space is used up. If you left your shadow copy on a small C:\ drive (SSD?) as is the default, it may not hold very many restore points, and you may have lost the one you need to get back to Win 8.0 by the time you decide to bail on 8.1. Restoring to an earlier version of 8.1 will not help. A disk image of the Win 8.0 system is the safest way to ensure that you have a working copy of your old system. The restore point system is a very complicated operation, and MS does not say that you can use it to get back to Win 8.0 from Win 8.1. I suspect that in many cases it will fail.
Refresh (which is the way MS says you can go back) will take your system back to the place it was when Windows was first installed losing all subsequently installed programs drivers and settings. "Personal" (data) files will not be erased.
Reset will take you back to your old system as it was first installed, losing all of your subsequently installed programs drivers and settings. It is equivalent to using the restore disk or partiton supplied by computer makers. All files will be erased by the format which is part of the operation.
Another question that is not clearly answered is whether it will be possible to move an optimally tweaked Win 8.1 preview system to the Win 8.1 final release version, or whether you will need to re-install and re-optimize a working system when the release is availble this Fall. I believe this was a problem for users of the Win 8.0 preview.