Bogus! I heard it too and any jet would have been closer to the Earth than to the Space Shuttle. This was just a terrible accident. Tinfoil-hattery is really not appreciated at a moment like this.
When I heard the report of there being a plane in the air, I didn't get the impression that it was being related to the shuttle in any way, but rather that the plane may have been at risk from the explosion and breaking up of the shuttle. Obviously, a plane would not be flying at that altitude and speed.
That said, there are some folks who will break out the tinfoil over anything at all. If a few didn't show up, we would worry about them.