I started up Eclipse today, created a new project, and immediately had an error before I had changed a single line:

Error generating final archive: Debug Certificate expired on 11/12/11

When installed, the Android SDK generates a "debug" signing certificate for you in a file called "debug.keystore". Eclipse uses this certificate rather than forcing you to create a new one for every project. In older versions of Eclipse, the certificate was only valid for 365 days, but as I understand it the default has been changed to 30 years in newer versions.

If for whatever reason you don't want to upgrade Eclipse, you can manually delete the certificate to for Eclipse to generate a new one. You can find the location in Preferences -> Android -> Build -> Default debug keystore (mine was in C:\Users\myUserName\.android\); just delete the "debug.keystore" file, then go back into Eclipse and Clean the project to generate a new file.