Last night a Macworld reader emailed me, asking how he could get rid of the "Buy .Mac" splash screen that shows every time he launches iWeb. I hadn't ever seen this screen, as I do have a .Mac account. So I blanked my account and password in the .Mac System Preferences panel, and then launched iWeb.
Sure enough, there's a big ".Mac Information" splash that gets displayed soon after you launch iWeb. It has three buttons; one to sign in, one to learn more about memership, and one to close the dialog and get on with using iWeb. After I closed the dialog, I checked the (very basic) Preferences in iWeb, and saw no obvious way to disable the splash. I then went so far as to use this hint to display all the possible defaults values for iWeb, hoping to find something like DoNotShowSplash or somesuch. No luck with that, either.
So I returned to the .Mac System Preferences panel, intending to just give up and write back "sorry, you're stuck with it." But then inspiration struck.
Instead of putting in my .Mac username and password, I entered a completely nonsensical username ("zxcvxzq2" in this case) along with a meaningless password ("123"). After typing the password, I pressed Tab again (as there isn't an "OK" or "Done" button or anything else). Below the password box, a small "validating password" line appeared, along with a spinning gear icon. In short order, .Mac came back and told me (as expected) that my name or password was invalid. After doing this, the login portion of the .Mac preferences panel looked like this:

Mac OS X Hints
http://hints.macworld.com/article.php?story=20060223074129928