Oops. You're right. For some reason the service didn't publish as Web Mercator. I had reset the data sources in my .mxd to the geodatabase replica, but they maintained the definitions in the layer properties for some reason. So I've started with a fresh .mxd and re-added my web Mercator layer, saved and published. Now the map service looks correct, showing Spatial Reference: 102100 (3857).
I guess I should know by now that 2229 is my local coordinate system, my bad.
Thanks so much for your help Robert.