The version of Dropbox from the Mint repos is not compatible with the new nautilus 3.x, which is why it wants to remove dropbox. If you allow that to happen, then you should install nautilus-dropbox
from the testing repos. This version has been updated to be compatible with all the Gnome3 goodness you have just updated to.
P.S. Don't worry about losing your settings in the dropbox uninstall, everything will be saved and exactly as you left it after you install the nautilus-dropbox package.
AMD Athlon II X4 630, 4 GB DDR3 RAM, Nvidia GeForce GT 430 - 64-bit LMDE (Gnome3) tracking testing, 64-bit Archlinux, 32-bit Windows XP
Aluminum Unibody MacBook - MacOS X 10.7 "Lion"