To access the offer, current DreamHost customers need only to go to the new Partners Page in the DreamHost control panel, then click on the Bandwagon logo to be taken to the Bandwagon signup page with the coupon code already inserted. The rest of the setup is pretty straight-forward, though I think I should share the solution I developed for a slight snag I ran into: For whatever reason, Bandwagon's FTP client couldn't log into my DreamHost account to begin uploading my iTunes library until I created a brand new FTP user. I don't know if this will hold true for everyone, but it's probably a good idea from a security standpoint anyway, take that advise how you will.
So far, after finally signing up for Bandwagon and getting everything going, I'm pretty happy with the service. Bandwagon's account manager can display statistics of what kinds of data and how many you have stored online. I'll have to see how the rest of the upload process on my 6396 item iTunes library fares before I say much more, but until then this sounds like a great deal that DreamHost customers shouldn't pass up.