The facts don't add up to an IS attack at the moment.

The aircraft was at 31K feet when contact was lost, and air traffic control radar analysis has highlighted a rapid drop (less than 30 seconds) in airspeed from 400+ kts to around 60 kts, followed by loss of track.

(reference the innterview towards ther bottom of the page)

http://www.bbc.com/news/world-middle-east-34687139

Although more detailed analysis of the flight recorders are needed and there are conflicting reports of whether the pilot asked for vectoring to a divert airfield just before the aircraft went down, a surface-to-air missile does not look like a culprit.

MANPADs possessed by any Sinai insurgents wouldn't be able to even strike an a/c at that altitude.

I suppose a bomb could be the culprit, and it wouldn't have to be a Hollywood-style endeavour to bring a plane down. There is definitely a lot of security theater at the terminal and a timer-based device could be just as easily emplaced through cargo access, with a bit of effort.