The roof-top parking area on terminal B is still closed, and I'm not sure about terminal A. Terminal C is open. I don't think that security guards frequent the parking areas, so spotters should be O.K.
I was last there a week before Thanksgiving heading out of town for the holiday so with some time to kill, and before going through security I decided to head up top and do some shooting sine the weather was so nice. After about 45 minutes or so and shooting from the roofs of A, A/B, B. I finally managed to make it to C, this is when I had my friendly encounter with an HPD officer. She had received a report a suspicious person (me) photographing from the roof of term C. After checking my D.L. as well as a few pics that I carry for just such an occasion, she said that "I hope you understand why I am doing this" and wished me a safe trip and drove off.
Of course this was before the attempted shoot down of the Arkia 757 in Mombassa so things may or may not have changed since then.
Actually, parking B is open for operations as this photo shows taken the same day that I was approached by HPD.