That is, according to the non-partisan National Journal, which puts out this Most Liberal/Most Conservative list every year.  In case any of my progressive friends out there were listening too much to Bill Clinton ("Obama's a Reaganite!"):

Sen. Barack Obama, D-Ill., was the most liberal senator in 2007, according to National Journal's 27th annual vote ratings.

Overall in NJ's 2007 ratings, Obama voted the liberal position on 65 of the 66 key votes on which he voted; Clinton voted the liberal position 77 of 82 times.  Obama garnered perfect liberal scores in both the economic and social categories.