For what it's worth, I signed up for a Dropbox account in late December, and have not received spam at the unique address I provided.
Re:Why was my email leaked?
by zacksjoden on Thursday February 28, 2013 @06:02PM
(#5301702)
I've signed up since then and haven't received any spam at all(also created a new email at the time).
If not, it may be that the compromised list of addresses from summer of last year has finally reached evil hands.