Yeah, I too notice the same problem with the back button not working properly. Actually in my experience it doesn't work most of the time. For example I just noticed it on Yahoo!'s tv listing site:
http://tv.yahoo.com/
Also there seems to be an issue with pages that either don't specify a title (in which case OW should just use the URL) or non-HTML documents (again, URL should be used) and OW decides to give the window a title of "OmniWeb"
I am using the latest, 5.5 sp10, BTW, on a Quad G5 on an NFS network account ( ~ is in /Network/Users)
-rory