That's Nikolai the Walrus, who apparently likes to play peekaboo with his zookeeper Bert van Santen, who has a surprise for Nikolai for the World's Animal Day. (Surprise, surprise, it's fish!)
Corrected Link Who knows that the link to this is - it changes by the hour!
Update 10/23/06: Turns out that the walrus' name is Natasja. There is a walrus named Nicolai, but he can't sit up! Thanks SÃ©verine Hendriks!