The official Wood Elf
FAQ states that it can only be used at the end of the
WE player's movement phase.
Note that this is a change from the first version of the
FAQ- when they initially released it, the
FAQ actually sanctioned using it in your opponent's movement phase. Unusually,
GW realized their error very quickly, and released a whole new version of the
FAQ within a week, which was identical in every regard except for reversing that one broken ruling.
http://www.games-workshop.com/gws/content/article.jsp?community=true&catId=cat210002&categoryId=500004&pIndex=2&aId=3000006&start=3