I know many, many people who have served in Iraq, worked in Iraq providing aid, contracts, or working on Iraq issues inside and outside the government over the entire course of the war. Not one of them thinks the Lancet numbers can be correct. As Jon Pedersen, the lead researcher for the UNDP household cluster survey conducted in 2004 (the only other, and much larger, study to use the Lancet method) recently stated, the Lancet numbers are "high, and probably way too high. I would accept something in the vicinity of 100,000 but 600,000 is too much"
I have seen a quote along these lines before, but can't find an authoritative citation. The link given does not work for me.