Expiry of the Debug Certificate
The self-signed certificate used to sign your application in debug mode (the default on Eclipse/ADT and Ant builds) will have an expiration date of 365 days from its creation date.
When the certificate expires, you will get a build error. On Ant builds, the error looks like this:
debug:
[echo] Packaging bin/samples-debug.apk, and signing it with a debug key...
[exec] Debug Certificate expired on 8/4/08 3:43 PM
In Eclipse/ADT, you will see a similar error in the Android console.
To fix this problem, simply delete the debug.keystore file. The default storage location for AVDs is in ~/.android/ on OS X and Linux, in C:\Documents and Settings\.android\ on Windows XP, and in C:\Users\.android\ on Windows Vista and Windows 7.
Source: Android Publishing Guide:
http://developer.android.com/guide/publishing/app-signing.html