Zaxwerks 3D Flag (which Michael linked to) is a great choice.
If you are using CS5, you could try Digieffects FreeForm (now bundled with CS5, available separately for older versions of AE). FreeForm is immensely powerful, but its controls will not be as intuitive for this specific effect as 3D Flag's.
A third option is Boris FX BCC Layer Deformer.
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