Directamente del manual de exec():
Cita: Note: If a program is started with this function, in order for it to continue running in the background, the output of the program must be redirected to a file or another output stream. Failing to do so will cause PHP to hang until the execution of the program ends.
Si quieres saber como hacerlo:
http://www.tuxfiles.org/linuxhelp/iodirection.html