John Lee Anderson has done an extensive profile of Iyad Allawi for The New Yorker, and, no, there is no new incontrovertible evidence that the interim Iraqi prime minister shot seven prisoners (one survived) in cold blood last year (as reported by the Sydney Morning Herald). Anderson does quote one of the alleged witnesses to the killings, and a "well-known former government minister [who] told [him] that an American official had confirmed that the killings too place, saying to [the minister]: 'What a mess we're in–we get rid of one son of a bitch only to get another."
Personally, I think Allawi perfectly capable of shooting people, as any successful Baathist official, present and former, is likely to have the skin of a crocodile; and if Allawi didn't do it, he's most probably not averse to letting such a rumor spread in Iraq anyway, to come across as someone ruthless.
Among the interesting, entirely parochial Lebanese tidbits in the piece is that Allawi's mother hailed from the prominent Lebanese Shiite Osseiran family. Lebanese Shiites have enduring relations with their brethren in both Iraq and Iran, which underlines how what goes on in Iraq, particularly a successful election come the end of January, will have a significant impact on other countries where Shiites live, through the community's familial and other networks.