Safari 3.1 was released today, and it promises vastly improved speeds. (I did a quick test with a CSS testing app and it went from 16ms to 12ms so it's off to a promising start). However, it breaks one of my favourite programs, 1Password. However, a quick fix has already been found.
First of all close down Safari and 1Password (print this out as a PDF or use another browser to look at this page).
Now locate the 1Password app in the Applications folder. Command-Click on it and select 'Show Package Contents' (quick Mac fact: All Applications are folders in disguise).
Head into Contents/Resources/SupportdBrowsers.plist.
Command-click and select Open With and Text Edit.
Click on Save and open 1Password, click on Install or Remove Browser Extensions and make sure that Safari is ticked. Now open up Safari and the familiar 1Password button should return.
Thanks to Initech for this hint. More info can be found here