it depends on how big the crack/break is. in my case, it's a relatively small crack, so it's not out of the realm of possibility to repair it.
essentially what they'll end up doing is slathering epoxy onto my toptube, around which they'll wrap a sheet of carbon fibre. of course, if you go to a professional, they won't do such a crude job.
i have a friend (a serious cyclist, to boot) studying chemical engineering who's in taking a class on composite materials right now, and he offered to do it as an experiment of sorts. i've declined to take him up on his offer, though!