I had thought this was self-evident, but by the e-mail I’ve received this morning it obviously is not:
When you see a ‘This beta has expired’ message, it means the beta version you are using has expired and you need to download a later beta.
This has been well covered in the forums. In addition, the expiration date is listed here in the dev-blog release announcements (and linked to from the news announcements). Also, an e-mail is sent to the beta mailing list with this information on every beta release.
If you are running the betas, you really should be on the beta mailing list. Beta software can have bugs and if there is a potentially serious bug, that information is sent to the beta mailing list.