This is now moot.. Dropbox is now in portage!  I'll keep this here just for prosperity :)

There is no ebuild in portage for Dropbox.  However, there is an ebuild out there.  Here's what to do:

Go here http://bugs.gentoo.org/237433 and download the latest ebuild.

The latest one today is nautilus-dropbox-0.6.1.ebuild created on 2009-05-11.

As root:
Copy the file to /usr/portage/gnome-base/nautilus-dropbox/ You will need to create the destination folder.
*NOTE* The destination folder has to be the same name as the ebuild.

Now run "ebuild /usr/portage/gnome-base/nautilus-dropbox/nautilus-dropbox-0.6.1.ebuild digest"
This should pull down the tarball of source code.

Chances are you will also need to add dropbox to your keywords file.
$ echo "gnome-base/nautilus-dropbox" >> /etc/portage/packages.keywords

Now just emerge the app as usual.
$ emerge -av nautilus-dropbox