This is the icon for Track, an iPhone app to manage your to-dos on Basecamp.
At first I was against using another checkmark, but the fact is, if you want users to easily identify the app on their home screen, you don't have much of a choice.
I used green on white because it doesn't seem to be used that much for icons. This makes it stand out a little bit more.
More about the app here: http://88bits.com/track/
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