Does the 2011 iMac have SATA 3 support?

It turns out that there is SATA 3 on board, it just wasn't enabled until 4 May 2011. The Other World Computing blog has the details:

While iMac EFI Update 1.6 is described as including “fixes that improve performance and stability for Thunderbolt,” it would also seem that an unadvertised benefit is that it also unlocks the full 6Gb/s, SATA 3.0 capabilities of two of the internal drive bays.

Unfortunately, the optical bay remains at 3Gb/s.

I don't have a 2011 iMac, however you can trust the statement from OWC that you linked in your question.

They bought one, took it apart and unequivocally state that there is NO SATA 3 support.

Since they sell the Vertex 3 and the industry is moving towards SATA 3, it would not be in their commercial interest to make this statement were it not true.