I can actually confirm the opposite: works fine here.
Whatever goes wrong? Do you get a “Z-Depth” indication in the file info? Can you do a 3D channel extract? (From my experience, typical size XSI renders require a setting of about -20…20 for the White Point and Black Point).
If you wish, drop me a line at randomgraph (at) yahoo (dot) com and I’ll take a look at your renders.
– Alon