Another angle on this is the following, obviously cleanly having just v5.1.7 is the best, but that will be tough for alot of folks. If you have Lion the following method is relatively painless (Mountain Lion removed the OS X RSS supporting code so Safari v5.1.7 won't run on Mountain Lion).
Leave v6.0 where it is.
Find another installed Safari v5.1.7 source (time machine, or a Mac that wasn't updated - Snow Leopard Macs did not get the v6 update). For a Mac that wasn't updated go into the Applications folder copy the package (its the Safari Application with the Safari icon in Applications directory) - it should be about 40MB in size if memory serves.
On your Safari v6.0 Mac, go into your Applications folder and create a new folder (I called mine Old Safari), then drop your Safari v5.1.7 copy into it. Copy the Safari v5.1.7 to the dock so you can launch v5.1.7 easily (I deleted my v6 Safari icon in the Dock but its not required).
Run v5.1.7 and enjoy its functionality again (its what I'm typing this in and others have used this method) and any updates that come for v6 should find the installed v6 and update it.
Important, do not run both versions at the same time (they use the same supporting files and corruption is likely) and just to avoid issues if you are running v5.1.7 I'd avoid using v6 till your ready to give up the ghost on v5.1.7 (who knows what kind of issues could raise their heads with them both using the same supporting files).
At some point, with this method, you could do an update from Apple that updates the underlying supporting code Safari uses and breaks Safari v5.1.7 (like disable the Lion OS X RSS functionality) so that's an ongoing danger, but this is an option for folks that want to use v5.1.7 again and can't roll back their machines.