The thing is, because the plane was flying at 36,000ft, there are only two things that could have shot it down; a jet, or AAA, possibly this -
http://en.wikipedia.org/wiki/9K22_Tunguska or this -
http://en.wikipedia.org/wiki/Buk_missile_systemYour typical MANPADS (Man portable air defence systems) have a max range of about 4,500m, so even if someone fired one straight up, it wouldn't have a chance to take down something flying this high. MANPADS were supplied to the Mujahideen by the Americans to fight off the Soviets in the 80s, as a direct counter to the presence of HIND helicopters etc, and were highly effective.
The Tunguska is an absolutely incredible, pants ****ting inducing bit of kit. It can fire 30mm rounds at 5k rpm, and their newly upgraded missiles can hit a target 10km away (32,000ft), so it is very unlikely, but still possible that one of these could have shot it down. These are Russian made, but 70 are in use with the Ukranian army. If separatists shot down this plane with one of these, it means the Russians have been supplying them with even more equipment than was initially thought, which could drop them even more in the **** with the UN.
The Buk is a radar guided missile defence system with a range of over 20,000m. However, reports are saying that this would have to be manned by a skilled crew in order to hit anything, let alone discern between a cargo plane and a passenger jet. Reports coming out over the last few days suggest that the separatists are actually in control of at least one of these, but they need to be operated in conjunction with a radar platform of some kind to work at all, so I'm withholding my judgement on that one.
A jet could have shot the plane down, but I think AAA is more likely, as a jet would have been able to make visual confirmation, and even if the pilot hadn't, then he would have realised that the thing on his radar was moving at a much slower speed than expected.
Just a military nerd's 10p