Bukkit

Check if the player is on the ground before allowing them to run the command. If Minecraft doesn't provide a simple player.IsOnGround() check, then I'd suggest checking their y velocity. And if that still doesn't work, get their height and check if there's a block beneath them.

Yep. Already had planned to do one of those things, it's on my to-do list. Might be a while unless someone tells me they've been having serious issues.