Hi Mark,
Reloading does not work for me. I can reload 10 times and nothing happens. I can click the external link 10 times and it opens 10 Safari windows and they all hang.
However, so far, as soon as I switch to the OTHER user, it works immediately, and then when I come back to my own window, it also works immediately. I have a feeling there is some shared url cache in Mac OS X which is getting hung up for some reason. Actually, even that makes no sense, because why would switching to another user have impact on the original user unless it was a shared cache, in which case, why wouldn't the other user hang as well?
Very confusing.
My next test will be to switch to the other user but NOT open the link there. And then switch back. And see if the act of switching users is solving the problem.
After that I will switch to my second user and click on a different link and see if that works or not. And if that impacts the original link on my user.
And I just want to remind you that this only happens to me with t.co links, which is twitter's URL shortener.
Strange stuff but the more information the better, in/when Apple takes a look at this.
JP