So if you've jumped up to Safari 3 on Leopard, you might have noticed that every now and then you're suddenly logged-out of various websites that you should have a persistent login to. Turns out there's a bug in Leopard's Safari (I don't seem to recall it being in the Safari 3 betas on Tiger) that wipes out your cookies file.
The reason behind it stems from a bug in Safari's handling of cookies when set in JavaScript (Slicehost has a quick write-up if you're interested about the details).
The quick fix? Quicksilver (or otherwise navigate) yourself to ~/Library/Cookies. Jump into Time Machine (you are using Time Machine, right?), click on the up arrow until you jump from a Cookies.plist file of more than ~20 KB or so (mine's usually around 600 KB), hit restore, and then restart your Safari session. Good as new.
It's a bug that's been mentioned more than a few times on the internet and it looks like there should be a quick fix for it, so I'm guessing this'll be remedied in the upcoming 10.5.2 update. Regardless, it's a great example of why Time Machine rocks so hard.