We can report that you can successfully and easily downgrade from Safari 5.1 to 5.0.5 under 10.6.8 by means of Charlessoft’s excellent (und for a long time well respected [I’m in now way affiliated with Charlessoft]) tool Pacifist (http://www.charlessoft.com/).
Just download this Safari 5.0.5 installer (for Snow Leopard) from Apple’s site (I hope it’s okay to put this link up here; of course Apple prefers you to download only its very latest – alas, the problems with this very latest is why you’re reading this post):
Then proceed as described here (look for the part named “Force Install”; don’t get confused that it mentions older Safari versions, like 5.0 and 4.0.5, the procedure is the same now):
Also see the concise documentation that comes with Pacifist.
Basically, in all the dialogues coming up just choose “Replace” (not “Update”), a choice clearly marked in Pacifist’s documentation for cases of downgrading. One restart, and you’re back to Safari 5.0.5 without touching any other part of your OS. And, of course, Safari 5.0.5 is supposed to work under 10.6.8, since it gets installed with the 10.6.8 Combo Update, so no surprise.
(Why not just use the above linked 5.0.5 updater, by double-clicking it? This updater denies to install 5.0.5 under 10.6.8, instead claiming to require 10.6.7 – but this seems to be due only to the fact that at the time the Safari 5.0.5 installer was current, 10.6.7 was the up-to-date version of Snow Leopard. And again, I’m writing this post – and, other than the one above which I wrote from within Safari 5.1, without any hiccups – from within Safari 5.0.5, which “flies” compared to my 5.1 experience under the very same, otherwise untouched 10.6.8 installation.)
Just one further caution: After the downgrade, Safari 5.0.5 cannot re-open the last session’s (the one under Safari 5.1) windows and tabs (something to be quite expected); so, in case, bookmark accordingly before initializing the downgrade. (And, of course, you do not loose any other information, like bookmarks, history, passwords, etc. The downgrade does not effect any kind of “brutal” reset of Safari.)
Finally, after the downgrade OS X’s Software Update does offer you the update of Safari 5.1. So, you clearly can get back again to the newest and latest if you wish to.
Here’s an alternative method, apparently even simpler (but obviously, you have to had installed Safari 5.0.5 before, otherwise you will go back to whatever Safari version you had actually installed before; you probably can get 5.0.5 by applying the 10.6.8 Combo Update after the downgrade):