So after extensive research and much trial and error, a team of dwarven scientists and I have isolated the gene that can give DWARVES the gift of FLIGHT (incidentally, it's just adding the [flier] tag to the raw entry for dwarves). There seem to be a few noticeable advantages and disadvantages to this genetic alteration.
Firstly, it's not so great for framerate. I guess 3-dimensional pathing is pretty intensive. I wouldn't recommend pulling this off in a large fort. At least, not without making a backup or something. Also, prepare for your announcement log to be FILLED (to the F***ing brim) with "Urist McDwarf cancels task: Dangerous terrain" messages.
Secondly, constructing tall objects no longer requires scaffolding, or careful planning to prevent dwarves from building themselves into corners. and not only is construction easier, but fortress layout can be completely revolutionized, with large, open shafts rather than stairs, and even areas of the fort completely inaccessible to non-fliers. Upon further review (based on additional observation and what was said below), it would seem that it is impossible to build special "flier-only" buildings, as dwarves will only fly-path to a place where they could normally walk-path. Moreover, it would seem that the "Urist McDwarf cancels task: Dangerous terrain" messages are not only an indicator that the game doesn't like it when dwarves fly, but it seems that a "flying dwarf" and a "falling dwarf" have very few differences. Essentially, a flying dwarf is unable to perform even rudimentary tasks while in flight, which seems to include drinking, planting, eating, fishing, and sleeping. In fact, it would seem that they're pretty much unable to do anything that doesn't involve combat, and even that, upon closer examination, seems to be littered with task cancellations.
Thirdly, sparring dwarves seem to spar in 3 directions...which is weird looking, but I imagine much more entertaining for onlookers than just normal 2-dimensional sparring. Additionally, Military dwarves can be ordered to patrol in a 3-dimensional route, which allows for myriad possibilities with marksdwarves, clever constructions, and perhaps lava.
Lastly, actual dwarf-on-non-flying-enemy combat seems to be now tilted in favor of the dwarves. Villagers can now easily flee goblin marauders in the up direction (though some still stupidly path along the ground at goblin-level), and actual attacks can, in some instances, resemble a hawk-like swoop onto the target (think hammerdwarves with jetpacks), or flying snipers, safely out of reach (provided they're smart enough to keep their distance in terms of z-levels).
On the whole, I would say that the experiment was a rousing success a perplexing yet thought-provoking failure, and I encourage any brave fortress-masters with a decent CPU and a penchant for the wacky to give this a shot. I'm sure I missed (and misinterpreted) plenty of what I observed in my brief foray into the word of dwarven genetic manipulation, but perhaps, at the very least, this should make for some interesting food-for-thought, if nothing else.
It would seem that until we can solve the flying=falling conundrum, dwarves will not be meant to fly.