Although I believe it's a decent possibility that Widmore is behind Dharma, I still think it may turn out that he is a newer player. In other words, Dharma could have been going for years before he got involved with it - assuming of course that he is.
I do think it's hard to trust that video that Ben showed. Assuming that's all there was to it, there was nothing concrete in it saying that the freighter was Widmore's. All we know for sure is that Widmore was taped where some guy was getting beaten up (or killed I guess). Not that it still couldn't be Widmore's freighter - it's just hard to trust what Ben says. In fact, when Locke asks Ben how Widmore knows about the island, Ben says "I don't know. But he does." I would think if Widmore is behind Dharma, that a) Ben would know that, and therefore, b) Ben would know how Widmore knows about the island. If Ben is telling the truth about Widmore's knowledge of the island then that would almost make me believe Widmore isn't involved with Dharma. If Ben is lying, then his statement that the freighter is Widmore's is just as easily a lie. The problem with Ben is that he mixes up lies with the truth so easily you never have any idea what to believe.
One thing that may point to Widmore being involved with Dharma is that the freighter people know that Ben used that gas once before to wipe out the Dharma people on the island. Of course, it's always possible he found out about it somehow without necessarily being involved with Dharma. But honestly it wouldn't surprise me either way.
Did anybody catch the book that Ben was reading when Locke brought him that food? Valis by Phillip K. Dick. One I haven't read but I looked up the summary on Amazon.com and it said this: "Known as science fiction only for lack of a better category, "Valis" takes place in our world and may even be semi-autobiographical. It is a fool's search for God, who turns out to be a virus, a joke, and a mental hologram transmitted from an orbiting satellite."