I've been to CDG
in June and in ORY
in July this year for spotting.
I had the gratis "spotting licence" which was very usefull. Although we were shooting quite far away from the oficial airport grounds we met 6 soldiers who checked our licenses. After they checked it they continued their patrol and we could continue spotting.
I don't think you need such a licence if you are only watching but as it is absolutely free I would order one.
we hd no problems with the airport authorities, neither on the observation deck nor at another spot close to the runway and far away from the terminal.
I hope this helps.
[Edited 2007-08-19 12:51:09]