I'm having a problem exporting hierarchies with transforms out of Maya into XSI. Even the most basic trees, sans textures, animation, histories, are coming into XSI scattered across the scene..

Apparently this is a known problem and just hasn't been dealt with yet? Unfortunately, in our case, it's pretty much a show stopper. So we're really hoping to see a little movement here in the near future. I've spoken with Guillaume Laforte on the Maya end of things and here are a few of his comments:
This is a known problem: it's a mis-match between the way the Maya Collada plug-in and the XSI Collada plug-in handle transforms. Since transforms are pretty basic to Collada, inter-operability between the two is sorta impossible right now.

The good news is that inter-operability is the big word right now for the Collada partners and we're currently discussing how to fix this mis-match, but it will take a few weeks before it is completely resolved, as it will involve changes on both side, re-testing, ... I'll keep you up-to-date on the developments, if you are interested.