XSI Mod Tool is a professional 3D modeling package with the ability to
directly interface with XNA. Unfortunately we have been unable to use
it up 'til now because of version skew: XSI Mod Tool requires XNA
version 2. As of today this seems to have changed. Version 7.5 has
been released, with the ability to interface with XNA 3.0. Take a
look at it here:
http://www.softimage.com/products/modtool/.