Luther did not hold to the concept of imputed righteousness?

I came across this interesting blog entry from Lee Irons: Luther and Melanchthon on Imputation

Irons states:

"I recently learned that many scholars think Luther did not hold to the concept of imputed righteousness. The scholars who belong to this camp are typically associated with the so-called “Luther Renaissance” that took place in the first half of the 20th century, mainly under the influence of the German church historian Karl Holl (1886–1926)."

Irons fleshes this out a bit, ending with the following Luther quote: "I think this, and am most persuaded and certain that this is the true opinion of the Gospel and of the Apostles, that only by a gracious imputation are we righteous before God." Interesting stuff, I'll have to dig into this when I get some time.