I can't tell you the source right now, but one major US retailer of Canon cameras reported that approximately 40% of their 5DMk2 sales were to video production companies. I don't remember if this was B&H, Adorama, or who, but it was one of the very big US names. I know many people personally too, who don't shoot stills, don't ever plan to, and only shoot video with their HDSLRs.
You've got to remember, compared to other 35mm & Vistavision sensor sized cameras, the 5DMk2 is ridiculously cheap. A lot of these production companies (especially in the USA) would think nothing of buying a dozen of them, along with a selection of lenses for each when compared to the cost of something like a single Arri Alexa, and if you're producing privately (corporate web and training videos, etc), as opposed to broadcast, you don't necessarily need an Arri Alexa (or even something as relatively lowly as an EX1 or EX3).