The Senate Rules Committee is currently holding this hearing. From the press release:

Serious questions have been raised about whether a photo ID requirement, when rigorously enforced without adequate alternatives, could prevent vulnerable voters from exercising their constitutionally guaranteed right to vote, one of the most essential rights in our democracy. In particular, concerns have been raised about the potential impacts on voters who have historically encountered obstacles at the polls — disabled, poor, minority, and senior voters.
The hearing will also examine the evidence of charges about rampant impersonation of registered voters at the polls, which is the primary type of fraud that could be prevented by a strict photo identification requirement.
Despite a major effort by the Bush Administration to investigate charges of voter fraud, and after millions upon millions of voters cast ballots in 2002, 2004 and 2006, no federal cases of impersonation voter fraud have been successfully prosecuted.