Google Earth 'Historical Imagery' Timeline from digitalurban on Vimeo.
While the majority of the imagery is currently US based, with some areas back as far as 1950, London can be viewed from 1999 onwards and other parts of the UK from 2001. It does make some interesting comparisons and could prove invaluable for the teaching of urban growth and urban sprawl.
The two images below are a quick example we found of new road and housing construction:
2008Change is obviously not limited to urban areas, it can also be of specific use in explaining changes to our physical geographic environment. The movie below
This is an extremely valuable update to Google Earth and one that will have notable impact in the communication and teaching of both physical and human geography.
Take a look at the Google Earth Blog for more features included in Google Earth 5 and Google's Lat Long Blog for more in historical imagery.