Until it is fixed you should be careful, what you put in your private galleries.
This line, entered in Google, gives EVERYONE access to a lot of PRIVATE galleries:
"site:http://www.dropbox.com/gallery"
I found 249000 entries with this line, and all the ones I tested directly pointed me to the private galleries of dropbox-users, including their account names.
Dropbox and Google are both informed of the issue, but it may take a while, so be careful.