It's important to remember that a missile that could destroy the ISS would NOT require enough delta-V to reach orbit. It has only to reach the altitude of the ISS while the ISS is passing directly over the area (and the ISS passes directly over most areas sooner or later). It could then disperse a huge cloud of shrapnel that would stay there for the critical few seconds required for the ISS to collide with the shrapnel cloud at orbital velocity. The shrapnel, of course, would immediately fall straight back down to Earth, so the timing of this operation would be extremely critical, and would probably require precision, high-tech guidance and control systems. Hopefully, this will keep such an operation out of reach of terrorists, even if they can get their hands on a small missile that could do the job.

Note that this type of attack (straight up and down) would give away the missile launch site, since a small missile would have to be launched from an area very close to directly underneath the collision point. A larger missile might be used to fly a ballistic arc with the top of the arc in the collision zone, which would allow a much larger launch origin area.

Fortunately, we (and several other large nations) already have a very large military infrastructure (including satellites) in place designed specifically to spot flying missiles and pinpoint their launch points. Unfortunately, we have nothing yet to stop them, but hopefully that will change.

So it would be very difficult to pull off such an attack and claim it was an "accident" aboard the ISS.

I'm not sure anyway why a terrorist would want to claim the destruction was an "accident," since that sort of negates the whole purpose of terrorism. The ideal terrorist act is one where the victims know terrorists did it, but can't quite pin it on someone.

Actually, the easiest way to take down the ISS would be to smuggle a fragmentation bomb aboard some payload or module, triggered much later by a radio signal or some other means. I sincerely hope that sufficient security precautions are being taken both here and in Russia to prevent this.