Long-term data encoding, Phoenix Mars DVD While browsing I've stumbled over the Phoenix Mars lander

http://en.wikipedia.org/wiki/Phoenix_(spacecraft)#Phoenix_DVD

and it says that the craft contains a DVD with all kinds of information on it.

Now my question would be: If the disc is made to last centuries or even millenia, how would one make sure that the digital information can be decoded in a future when all knowledge about present-day encoding must be assumed long forgotten?

I'm not even talking about building a working DVD player; if all else fails, take a strong microscope and extract the bits the hard way. 

It's about, how would I, as a future scientist, interpret the bits such that the original meaning is restored, and how would I know that I've picked the right decoding? "Digital Memory Loss" has been recognized as a major problem already, and it's only been a few decades since we earnestly started producing (and forgetting!) mass quantities of digital information.
Ordinarily, I'd think this was out of scope. However, there's an interesting series of papers involving Madhu Sudan, Brendan Juba and Oded Goldreich on what they call universal semantic communication (or how to talk with aliens). The papers are here: one, two, three.