Do you support Safari 5.1?
In short, yes!
The slightly longer answer is Safari 5.1 (on both Snow Leopard and Lion) is unreleased software and therefore is subject to many changes. We therefore only support it in the latest beta version of 1Password.
To learn about how to opt-in to help test beta versions of 1Password and get our new extension installed in Safari 5.1, please see this forum post.