My mistake, I have Safari 5.0.3 as well. However, I've done some more testing and it seems as if the "bug" only occurs when you're logged out of Twitter. Let me know if that's the same for you.
Ah yes, I can confirm that. It's totally hosed when you're logged out since it redirects to the logged-out search page, instead of the integrated one when you're logged in. It also breaks the parameter encoding when it redirects. Nice.
