It appears to be true, though I'd imagine they're getting very substantial discounts from Amazon. Otherwise it'd be far cheaper for them to self-host at their scale.
If that's true wouldn't it make sense for Dropbox to form some sort of a a whitelabel partnership with Amazon? Building cross platform reliable software to sync and method of storing data (ie. avoiding duplicates et al) is still an important piece in creating a smooth user experience while minimizing cost on the storage side.
