Say for example you have a complete seal (unlikely) between yourself and the toilet. Is the amount of suction possible to pull one's intestines (or insides) out and into the toilet? I think (probably a joke) on a NASA show that the toilets in space can do that, but probably just meant jokily. But, back to the original question, is it possible?
Well, while kind of an odd question, there does happen to be a pretty simple answer so...
The Mythbusters proved, in one of their episodes, that the pressure differential created by an airline toilet seat isn't even enough to make a person be stuck to the seat itself.
I think it's fairly common knowledge (or can at least be assumed) that if you wanted to suck someones guts out of their body you would, as a first step, need to be able to create a proper vacuum seal between that person and the toilet seat. Since it's impossible to create a proper vacuum seal with an airline toilet, we can deduce that that self same toilet lacks the power to suck out your organs.
So, if you were worried, don't be. Feel free to use that airline throne all you want ;).