This is something I've had conversations about.
Alot of wing chun is pushing forward (duh!) so you are using (if someone has better anatomy than me, please correct me) mostly the front muscles on the shoulder, and pec (as well as other, i.e. tricep etc), however, there is not much excercise of the muscles on back that pull the shoulders back.
This probably doesn't explain it very well - however, it is likened to someone who spends a long time in front of a computer - the shoulders come foward, and the back curves (especially if they slouch) as the muscles at front will tighten. Thus if you work in front of your computer for long periods you should do exercises to stretch out your pecs, and constrict your back muscles, to stop this from happening.
(I find bell flys, and dumbell rows etc really help this)
Anyhow - back on topic - without proper excercise to balance these effects, you CAN get a curved back and shoulders from doing alot of chi sau, because you are constricting muscles at the front, and not at the back.
All in all - its not a GOOD thing, as its bad posture, and bad for your back. So make sure you work out your back muscles when you do wing chun!
'If someone wants to fight you, run a mile. If they are still behind you after that, run another mile. If they still want to fight, and it is really worth it, turn around and beat the living !*$!% out of them, 'cause they will be really tired.'