Installing Safari 4 had a pretty strange effect for me: Lights in the house switched on and off at random.
It turns out that Safari likes to visit your favourite pages periodically to update its Top Sites browser. Which is fine unless some of the lights in your house are controlled by a web interface which uses GET for the light switch buttons.
Since it's private to the house network I hadn't bothered to password protect it. I assume Safari would have left it alone if I had. On a hunch I fixed the problem by switching the lighting control page to https - and it seems to have worked.
I can't help wondering 1) how many intranets it's attacked already and 2) does it know to refrain from clicking on sites that use cookie based auth (RT?) and if so, how?