it's been the rule for a couple of years at least that unless the ref explicitly says you may leave the pitch, you are counted as on it. The rationale is simple, and very reasonable - to stop someone 'stepping off' the pitch, or staying off it if you slid there, to put someone offside.
In this case, the ref seems to have gone with the absolute letter of the law, as chappy seemed to be genuinely injured, rather than playing at it (tho he was back up and in the game pretty quickly afterwards), so most refs would say he wasn't in play. But, technically, he was.
And, fuck it, even horse-face deserved that one, cracker of a game.
e2a: the beeb report -
http://news.bbc.co.uk/sport1/hi/football/euro_2008/7445476.stm - makes it sound like blokey
would have been considered out of the game if a dutch player had injured him, but as it was one of his own, its tough titty.