As the above poster has explained, this is an extension of the fact that invaders' mounts determine pathfinding (which also leads to such hilarity as goblin super-elites drowning as their mounts try to path through your aqueducts). But the awesome thing is that Toady won't have to do a damned thing to get semi-intelligent horse archers. I, for one, can't wait until I can't just send four axedwarves laden with three tons of steel armor to dispose of anything the goblins send at me.
Granted, there is the distinct probability that all the aforementioned shock troops will have to do to counter the goblins' mobility is to trap them in the corner of the map, and that seems distinctively unsporting.
Since when has"being sporting" been a dwarven thing?
Perhaps "sporting" is not the word, but Dorfiness as a virtue does preclude a certain kind of behavior. No self-respecting, Dorfy dwarf would be caught dead using a wall of cage traps as his only defense. The said cage traps, however, become permissible as long as the dwarf is planning on doing something suitably cruel, cunning, and/or impractical with the prisoners he thereby amasses.
Cage traps are unDorfy by themselves, but become Dorfy when the poor goblins are stored in a 100 z-level tower, to be dropped from the very top all the way down to the third cavern layer, their progress being halted 3 stories at a time by retracting bridges in order to maximize their pain before death.
Taking advantage of the ability to make physics-defying buildings for the sake of laziness or slovenliness (i.e. ugly, asymmetrical abortions of architecture), especially in order to make some aspect of the game easier is unDorfy. But building an utterly impossible orbital magma cannon, preferably supported by a single column of soap submerged in yet more magma, is VERY, VERY Dorfy.
Similarly, cornering the goblins' crack team of horse-archers in the corner of the game screen is unDorfy. However, making a needlessly-complex series of ditches and walls to achieve exactly the same end, especially if those ditches are filled with magma and the walls are made from the tallow of thousands of ducks, would be considered quite Dorfy.