Obama has achieved a virtual tie with Clinton according to a poll by American Research Group.
The poll of 600 likely Democratic primary voters shows Clinton with 34% and Obama with 31%, which is within the margin of error of 4%.
To identify trends over time, I plotted the results of Clinton vs Obama match-up results from various polls released between 12.9.06 and 3.10.07.
The graph illustrates Obama's impressive and sustained surge and that Clinton's static movement.
The poll data is from Angus Reid Global Monitor.