The Washington Post article cited below on the constitutionality of the DC Voting Rights bill references DOJ testimony in 2007 by John Elwood, then a Deputy at OLC. You can find the testimony here; it seems quite persuasive to me. For the contrary view, see Ken Starr's 2004 testimony here.

  UPDATE: My colleague Jonathan Turley as also written an article on this that was published last year in the George Washington Law Review: Too Clever By Half: The Unconstitutionality of Partial Representation of the District of Columbia in Congress.