I just figured out a way to get Google Docs working in Opera 9.51 and i tought i'd share it:
Install the Neptune plugin.
Then just choose Mask as Internet Explorer. (Right click, select "edit setup for web site"? then network)
Then it all seem to work.
Have a nice day.
25. juli 2008: Google Docs seems to work without the plugin again if masked as Internet Explorer.?