I dont know exactly when (Late July/Early August?), but Google has updated most of their Google Earth satellite imagery. They have not, however, updated any of the imagery at maps.google.com (except for a portion of Katrina-damaged New Orleans).
For example, here's Area 51 at maps.google.com:
And here's Area 51 as seen on Google Earth:
You can click on the pictures above for the full sized photo, and HERE's a link to the current Area 51 imagery at maps.google.com.
I also noticed that they've updated the Los Angeles area, the Boston area, and Washington, DC (And they're not even blocking out the roofs of the White House or it's surrounding buildings anymore).
Google Earth Hacks has a page with myriad .kml files that you can download and add to Google Earth that updates the imagery on your PC. Since you can click them on and off, you can go back and forth between before and after.
I think what you're looking for is under Tools > Internet Options > Temporary Internet Files > Settings. Check for newer versions of stored pages: set that to every visit. This is on Win98 IE 5.5, though, so caveat emptor or something.