The current issue with putting a limit on how many creatures can occupy a tile at one time is that it can cause a whole host of pathfinding problems. The way dwarves currently calculate their pathfinding is to just find the fastest route to the destination based on the current obstacles IIRC. If something is blocking the Dwarf's path to his/her destination, he/she just recalculates the entire route, acting as if the obstacle is completely unmovable. Now, this is fine because at the moment anything which blocks movement is impractical/impossible to get rid of. However, if there is a limit on how many Dwarves could fit into a tile, the pathfinder would just try to find a completely different route every time another creature tries to mozzy on through his route, requiring the pathfinder to just completely recalculate the route even if it would take a single step to the side to get around the creature. Without a rewrite of the pathfinding system as a whole, implementing a limit of creatures per tile would result in absurd numbers of lag (from pathfinding constantly having to redo every route when as so much as a baby lamb decides to take a stroll by) and job cancellations.
I totally agree that a limit of creatures/items per tile should be implemented into the game, for many reasons ( Combat more interesting, Hallway and traffic design is more important, easier to look at a fort in general, ect. ), but its not something Toady can just do offhandedly. Such an update would be a fairly major milestone.