Valid point and it crossed my mind too, however Google's systems are aware of domain shifts (I assume the Dropbox folks filed a domain change) and it should know about its 'transferred' positive rap.
If the block is solely based on malware distribution then Facebook would have suffered the same fait ages ago.
Amisdst all the data Google stores it should be clear that Dropbox isn't a random site freely dropping malware around the web.
If it can't figure it out then a good old fashioned human should.
