First thing I did when I returned from Australia was to run software update on my three computers. This included the 10.6.2 upgrade to the operating system. All went well, but then I noticed strange activity on one computer, the two-year-old iMac 24in. Several of the logon items had not appeared on the toolbar; then various applications wouldn't load. The icons bounced for a moment but nothing more happened. Most of the built-in applications such as iCal and Mail worked as normal.
Although I had seen this happen before, I'd forgotten the cause and remedy, so I posted a thread on Apple Discussions. If you haven't explored Discussions before, it's worth a look. In my experience, answers and help come within hours, if not minutes. There are so many experts out there, most of them specialising in one particular aspect of Mackery or software.
In this case, as usual, I was not disappointed. The first suggestion was that I download the Combo update for Snow Leopard and install it before doing anything else. This I did and it cured the problem immediately. Some settings files had obviously been corrupted by the latest update. The Combo installation cured this and it is always worth trying this if there are strange, inexplicable happenings.