But the Dropbox developers have finally released Dropbox .deb files and added a Dropbox Ubuntu Karmic Koala repository.
To add the repository, simply run the following commands:
sudo sh -c "echo 'deb http://linux.dropbox.com/ubuntu karmic main' >> /etc/apt/sources.list"
gpg --keyserver pgp.mit.edu --recv-keys 3565780E
And then install it:
sudo apt-get install nautilus-dropbox
Update: Dropbox has changed it's repository so I have updated the post with the new address.