I am running 1Password 3.5.7 (build 30884) on OSX Lion 10.7 (build 11A459e) Developer Preview 2.
I have found a reproducible exception in 1Password.
Steps to reproduce:
1) Launch 1Password
2) Click on "Software" on the left pane
3) Click on an entry on the top right pane
4) Click on the "License Key" for the entry on the bottom right pane
5) Press command-tab to change windows
At this point 1Password should pop up the exception that I have attached to this post.
I realize that Lion is still in development. This was more of a heads-up for the 1Password developers that this bug exists.
Apologies if this isn't the proper place for this bug report -- I looked around but didn't see any place to report bugs with 1Password and Lion.