Just installed on my PC running Ubuntu Server 10.04, a little hard to find source to add to repository. But here it is, all you need to do is just follow these steps:

1. Make sure you have installed Nautilus

$ sudo apt-get install nautilus

2. Install public key to authenticate Dropbox's Ubuntu repository

$ sudo apt-key adv --keyserver pgp.mit.edu --recv-keys 5044912E

3. Add source to repository

$ sudo vim /etc/apt/sources.list
#Dropbox
deb http://linux.dropbox.com/ubuntu lucid main

4. Update repositories

$ sudo apt-get update

5. Install Dropbox integration for Nautilus

$ sudo apt-get install nautilus-dropbox

6. Starting Dropbox daemon

$ dropbox start -i

You have to restart all intances of Nautilus, then start Dropbox and fill in your account information. That's it! :)