Avocadoes contain persin, a fungicidal toxin that is poisonous to a lot of big mammals and birds.
I couldn't find definitive answer about reptiles, probably because reptiles and food-growing plants don't usually interact.
There is a post on Reptile Forums about avocadoes being dangerous, but after reading it I don't recall anyone actually providing any evidence that it was dangerous to reptiles. One user says that they found out avocadoes were a big "no" for bearded dragons, but doesn't cite anything.
I'll do more research and edit in what I find, but I would probably not put the tree in a viv. At least, delay it until we know more.
: A website lists different foods toxic to turtles and lizards: http://petcaretips.net/poisonous-pla...s-lizards.html
I don't completely trust it, as it doesn't look especially academic, but it's something.