I just saw this on Wimberley’s site, apparently the new Arca Swiss Z1 ballheads, at least those produced between some undetermined time and June 15, 2008, are composed of a 2-piece shaft, that is only glued together. This can result in the top of the shaft and clamp breaking away from the ball potentially causing your camera to fall.
Arca Swiss has apparently addressed the issue in their manufacturing process now, but I have no idea when the current production stock will be cleared out and the fixed models will be available.
I’m still recommending this ballhead, as it’s arguably still the best out there.
See the link below for the Wimberly page discussing the Arca Swiss Z1 compatibility with the Sidekick.