http://www.soadfans.com/Article514.htm
John Dolmayan joined TOOL for the last two nights helping the drummer Danny who recently had an arm injury. He stepped in on Lateralus the last two shows TOOL played at the Pearl in Las Vegas.
Taken from TOOL fan page:
"Maynard said something about 'a certain drummers arm' and somewhere around sober he asked Danny how the arm was holding up and Danny hung his limp arm at his side, kinda funny. Maynard introduced John from System of A Down before the 'Drum Battle' between him and Danny. Then at the end Maynard said 'see ya tomorrow'. He probably said a couple other things that I can't remember right off but he didn't talk much."
from the toolnavy.com