Title is a bit misleading. It's the site, www.dropbox.com, that is flagged. Which might be reasonable, considering that people can and do host arbitrary publicly accessible executables on dl.dropbox.com.
I actually had it complain about dl.dropbox.com also when grabbing a file for work this morning. That's what made me post it here. IE wouldn't let me download a file from "an untrusted site" without jumping through a few dialog boxes.
EDIT: It looks like it's gotten some attention from MS at some point as it seems to have cleared up now.
