I'm not sure if it means anything or not, but it would seem that shannon saw her walt vision as walt was being taken. Shannon sees walt before jack, locke, kate, and hurley return from the hatch. In tonight's episode they cut to sawyer pulling michael out of the water to locke letting kate down the hatch. They seemed to be showing the same events again to let us know the timeline of events. If we say it took an hour (insert you own time) from the time that shannon saw walt and kate going into the hatch, that would mean that the walt sightning happened an hour before sawyer pulled michael onto the raft.

What was walt doing right before the others came for him?

So, what was that walt sighting? The others interacting with the survivors? Perhaps whatever the execute button does is bad for others/voices and they let her see a vision of walt telling her the button is bad (albeit backwards).

This is all so very confusing, but that's why it's a good show.