I use SoqlXplorer daily. I just allowed it to update to 1.80. The new version boots, asks for a login, and when I select one that is stored in a keychain, it hangs. I've tried rebooting it, and I've tried directing it to a different sfdc instance (in a sandbox), with no improvement.
I downloaded 1.70 from http://www.pocketsoap.com/osx/soqlx/, and it does not exhibit this problem.
My environment is a MacBook Air, 10.8.2. Simon, let me know if you would like any other data.