What Do You Do If A Hippo Attacks?

I would say climb a tree (they may just knock it over) or crawl into a deep hole (they will still do their best to get to you).  If neither are available. I would stand still and pray.  You can't outrun a hippo and it will just make him madder to have to chase you.
 
The have two hippos at the Aquarium in Camden, NJ.  They have elaborate gate systems so they don't have to have an attendant too close to them.  Some say the hippo is the most dangerous animal alive.
 
They are supposed to be vegetarians but I saw a nature show where they were very surprised to see one chewing on a drowned zebra (or similar animal).
 
Aquarium website may have some information.

Poke it in the eyes

ONLY a guess so don't rate me on this one.... But some animals lose interest if you don't make eye contact with them....and freeze.  I've heard this said of attacking dogs. Of course if you're running it's likely you're not looking back anyway.

I'll just run away!
:)