  Differential studies of elliptic flow are one of the most powerful tools in
studying the initial conditions and dynamical evolution of heavy ion
collisions. The comparison of data from Cu+Cu and Au+Au collisions taken with
the PHOBOS experiment at RHIC provides new information on the interplay between
initial geometry and initial particle density in determining the observed final
state flow pattern. Studies from PHOBOS point to the importance of fluctuations
in the initial state geometry for understanding the Cu+Cu data. We relate the
elliptic flow data to the results of our model studies on initial state
geometry fluctuations and discuss how we will perform measurements of
event-by-event fluctuations in elliptic flow in Au+Au collisions.
