This is odd. I have to use View Options to see full headers (in the Message Options window), but while that's open, the main Outlook program is visible but completely unresponsive. Message Options is apparently a super-modal dialog, which blocks "other applications", and the Alt-Tab task switcher doesn't even show the main Outlook icon. I thought Outlook had crashed, until I realized it was accessible again after I dismissed Message Options.

In fairness to Apple, I've seen cases in Leopard where Apple's Command-Tab task switcher only shows some of the currently running applications (it sorts itself out fairly quickly), but Windows' super-modal behavior is fscked up, and designed into the application (or the OS!).