Well, I got to do only a minimal amount of slinging on my vacation.
I did give the second Black Mamba to my brother, but he's pulled in a lot of different directions, attention-wise, these days, so he didn't get around to taking a lesson. (I recently sent him a link to Brian Grubbs' Figure-8 video, though.)
I'm currently working on knitted sling #6; these things are a lot of fun to make!

So far, I've made the yellow one, a hemp one, the original Black Mamba, my brother's Black Mamba, another longer hemp one (42"), and now a third Black Mamba (~45").
Still haven't gone out with golf balls to try them out, although I did sling about 10-15 rocks into Long Island Sound on 9/11/10, and also about 20 into Nantucket Harbor a week later. I'm looking forward to using a 40-someodd inch Black Mamba with golf balls, really. That, and I'm going to be refining my technique more along the lines of Brian's. I realized that I haven't been doing the "step-into" portion of his Figure-8.
Perhaps this weekend, I'll find the time to get out to sling a bit.