Samsung Galaxy S III users on Verizon and AT&T won't get Dropbox deal
The deal actually gives users an additional 48GB of storage on Dropbox, but because you automatically get 2GB free anyway, it adds up to 50GB. But, the support page says that only Samsung Galaxy S III models that have Dropbox pre-loaded are eligible for the deal, and certain carriers have not opted-in to the deal. Only Verizon and AT&T are named outright as carriers that haven't opted-in to the deal.
However, if you happen to live in China, Cuba, Iran, North Korea, Sudan, or Syria, you also won't be able to get in on the extra storage deals.