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They have the [very cool] option to get directions by walking now, in addition to the regular "driving" directions. But it seems like it would be rather simple to add a "distance by air" to that drop down box. Furthermore, why have a drop-down-list-box for such few options?
Now, I know you can measure distance on Google Earth (but you have to install that), and it's been rumored that you can get something similar done within Google's "my maps" section of the site, but I've never gotten that to work. Let me know if you have! ;)