The latest version of WordPress (2.6, released today) includes Google Gears that stores "most of WordPress' images, scripts, and CSS files ... on your computer. This speeds up page load time."

http://wordpress.org/development/2008/07/wordpress-26-tyner/

I thought they would let you backup the database using Gears, what a bummer (I don't use WP anymore, but I do quite a bit of WP related work).

The WordPress folks have brainstormed about doing that, using Gears as a local backup. You should submit a patch ;)

Best,
   Brad Neuberg
   Gears