Not exactly an emulator feature but related.
When using Eclipse for Android development, the debug certificate that is automatically created only lasts 1 year. When it expires, it's a pain to have to delete all your development .apk files, because the new certificate doesn't match so you can't upgrade them in all your AVDs. You have to get another development MAP-API key as well. The published recommendation is to simply delete the debug.keystore file and Eclipse (ADT really) will create a new one for you, again with a 1 year expiration. But you can create your own debug certificate in debug.keystore with whatever expiration you want. Do this in the .android folder under your HOME directory:
keytool -genkey -v -keystore debug.keystore -alias androiddebugkey -storepass android -keypass android -keyalg RSA -validity 14000
ADT sets the first and last name on the certificate as "Android Debug", the organizational unit as "Android" and the two-letter country code as "US". You can leave the organization, city, and state values as "Unknown".
