Yet another example is MineCraft. The world is generated by a 32-bit seed, yet it is infinitely large (theoretically). Obviously, you can't reverse that back into a seed, as explained above.
I would like to point out that early versions of MineCraft used to save the differences between the original world generated by the seed and the modified world edited by the player. Alas, the longer the world existed and the more you mined, the larger the save-files became and the longer it took to apply the differences whenever you loaded the world.
I recommend saving absolute states and not relative states. You can optimize it in many ways, one way is only saving the sub-sections that are different than what was generated by the seed.