Imagine controlling your own cellphone based robot... all you have to do is plug your arduino into the usb port...
Now it can connect to internet wirelessly (the ultimate remote control), has gps, accelerometer, speakers (just think of the pranks you can pull with that), web camera, 16 GB disk, 256 MB RAM and about 1 Ghz for processing.
Unfortunately the android platform doesn't yet have USB host features... so when you plug your arduino card into the phone... the phone will just lie there staring back at you...
But there is hope for all you mad scientist in need of a robot army...
The above link will allow you to vote to show google you really really really need this... Make a gmail account you'll never use again and just click on the star in the top left corner...