The short answer is that Wilkerson is a retired U.S. Army Colonel who served as Chief of Staff to Colin Powell when he was US Secretary of State. But more recently, his name is turning up in some awfully strange contexts.
A report in Haaretz quotes Wilkerson as alleging that the use of chemical weapons in Syria might well have been part of an Israeli "false flag" operation, designed to justify Israeli attacks. Obviously, this is hardly a mainstream view; even the President's now-famously-reneged-on "red line" formulation reflects a belief that Assad has chemical weapons and a wllingness to use them. Interestingly, Wilkerson surfaced to make his charges -- and to call Benyamin Netanyahu an "idiot" -- on nutty-left Current TV, which Al Gore famously sold to Al Jazeera. He actually seems to be something of a regular there, and has appeared on Democracy Now to blame Dick Cheney for Colin Powell's UN speech indicting Iraq for having WMD.
Wilkerson is also part of the ironically named Military Religious Freedom Foundation, the group hostile to religious expression in the military, as discussed on this blog last week. As such, he's working with the intemperate Mikey Weinstein, the guy who has denounced the "fundamentalist Christian monsters" in the military (so much for the left's oft-touted calls for "civility").
Oh, and Wilkerson's also the guy who claimed that the GOP was "full of racists" during last year's presidential election campaign.
Pretty interesting commentary on Colin Powell that this guy was his chief-of-staff. He now seems to be something of a convenient mouthpiece for the left -- though he claims to be a Republican.