If you run into weirdness with the Facebook or Flickr Uploaders in iPhoto '09, try this. Keep in mind that if you share your Snow Leopard account and someone else is signed into their Facebook, Flickr, or Yahoo! accounts under your SL account, you'll have problems. Try logging them out and yourself in before deleting these keychains.
I found this post last week when I had the same exact problem. Today, I finally just did a Google search and found a post in the Apple discussion threads (http://discussions.apple.com/thread.jspa?messageID=9936676#9936676). It says all you have to do is go into the Keychain Access application on your Mac and delete the iPhoto key from your computer.
Just search for "iPhoto" in Keychain Access, and if you have more than one key for iPhoto permissions (I have Facebook and flickr), just double click them until you find the one that contains "flickr" in the "Account" text box and delete it. I just did this, and after I restarted iPhoto, when I clicked the flickr button in the lower right, it asked me if I wanted to setup iPhoto to publish to flickr. AWESOME.[From Flickr: The Help Forum: iPhoto '09 flickr uploader Permissions]