From what I understand, the default speed for a creature is 800, meaning that all creatures wait 800 'frames' (these not being graphic frames, but more basic steps taken by the engine). Seasons, presumably, have a set number of simframes per day, month, and year, and a number of other calculations for everything from hunger to goblin sieges and many other events rely on that basic system, waiting a set amount of time, then acting, then waiting. From that, it seems that by adding a control to fortress mode that would universally change these values, and thus the amount of time spent waiting, could allow a game speed control. The great benefit of this is that people with weaker machines can sacrifice a very small amount of granularity (if the game is sped up by 200%, creature A that is one point faster than creature B is rounded down to being the same speed. this is very rare, however, and should not be objectionable at any reasonable value) and in exchange can run the game at the same GframesPS and simframesPS, but have seasons advance and constructions complete at twice the speed! Since time is more flexible this way, some actions that should take more time than they currently do, such as some digging, crafting in general, etc., can now work at a more realistic rate, since the player can just "fast-forward" through the boring part!
P.S. 'Realism' being in context with the game world, as I understand that goblins and undead are somewhat unrealistic...
[ December 10, 2007: Message edited by: PTTG?? ]