Every so often, I see a post in here asking about imagery updates in GE. While the short answer is always "when Google updates it", I thought some of you would like to know that DigitalGlobe has a subscription service. They offer to send you EMail alerts when they update imagery in regions/areas of interest of your choosing.
DigitalGlobe Subscription Center
I have an existential map; it has 'you are here' written all over it.