Apple just released a beta version of Safari 4 with lots of new features. Users of 1Password might notice that it doesn’t work in the new version. To fix this, close Safari and 1Password and edit the following file:
Find the Key named Safari and look for MaxBundleVersion underneath. You will see 5528.1 as the maximum version. Change that to 5528.16, save and quit. Now you can reenable Safari from the 1Password preferences and then start Safari 4 and add the button to the toolbar. Voila!