Many Dropbox users are reporting problems with installing Dropbox under GNU/Linux based systems such as Ubuntu or Debian. When you try to install Dropbox from the repository using nautilus-dropbox package, the installation will be stuck at 99% when installing the daemon.
A Dropbox employee, David, wrote on Dropbox forum:
On Ubuntu and Debian we've changed our package name and versioning to not conflict with their packaging of nautilus-dropbox (which by the way is unsupported by us). Basically if what you have installed is called nautilus-dropbox then it is the incorrect package. Please uninstall it and go to www.dropbox.com/install and download the latest package (dropbox-1.4.0 at the time I wrote this) and install that.
In order to install Dropbox on Ubuntu or Debian based systems, visit the Dropbox download page and download the binary for your system and install manually. As David suggested if you already have Dropbox installed, please uninstall it before trying this method.