I just bought the Samasung Galaxy Ace and experiencing the same problem.
When i pull down my notifications bar (When plugged into laptop or when not) it only shows me:
Wifi-B/t- GPS-Silent- Auto rotation
Im convinced i have seen the USB button somewhere on the phone but i cannot find it!
Ps. any idea how to remove the multipule windows from the main screen? I can slide along to 4 but i only need 1