For those that are still using the Mac OS X Public Beta as your primary OS or on a test machine, that version of the operating system will expire today. The May 15th expiration date was originally announced when the Public Beta was released late last summer, which essentially set todayis date in stone as the deadline for delivering OS X 10.0. Apple beat that deadline by well over a month, and those that have continued to use Public Beta and not made the switch to OS X 10.0 will not be able to use the Public Beta after today. According to an Apple TIL article (n60845):

Mac OS X Public Beta has an expiration date of 2001-05-15. You should upgrade to Mac OS X if you use Mac OS X Public Beta. Mac OS X offers reliability and a rich assortment of features. For more information on Mac OS X, please see www.apple.com/macosx/.

Mac OS X Late Breaking News (available on the Mac OS X Dock -- after Mac OS X is installed) contains additional information on upgrading from Mac OS X Public Beta.

Note: The Mac OS X Public Beta software license agreement also states this expiration date.

Important: When upgrading, you should erase the hard disk volume containing Mac OS X Public Beta. If you do not do this, you may experience issues with your network settings and fonts. Before you install Mac OS X, back up any documents you have on your existing Mac OS X Public Beta disk or disk partition.

You can find more information on Mac OS X, and read the full TIL article, at the Apple Web site.