David Troy has built a nice Twitter/Google Maps mashup that shows Twitter updates on a world map, as they are posted (with a delay, depending on traffic levels):

The green and orange buttons let you indicate whether or not you like the particular update. I don't know what their effect is, though. (Any ideas?)

It also provides an alternate view, where you can see where a particular Twitter user has posted from.

Lastly, in true Web 2.0 spirit, he even provides his own Twittervision API. An API for an application built on the Twitter and Google Maps APIs. How very "meta"!