I'd not met any in the flesh before, but yesterday I met one lady who tandems and one who did - and, I think, who tri-andemed at one point. We weren't even at a baby-related group, so it was a bit of a coincidence.
I feel dead chuffed. I'm likely to be tandemming myself unless DS or I change our minds before DC2 arrives, and it always feels odd to be doing something that no-one you've ever met has done.
There's no real point to this post, but I felt I needed a forum to say how pleased I was. :)