various things come up in this forum if you search from google, key words being "evaporator drain lexus is250"
http://us.lexusownersclub.com/forums/index.php?showtopic=53610
I had the same problem on my 07 IS found out there is a plug you have to pull out to let the water drain. There is a bulletin on this problem
the dealer had to pull the trans to get to the plug.