Hi Steve,
They look good, but I should imagine the texture alignment would be lost on conversion, and the alpha textures would not appear in elevs/section, just the crude outline of the 'branch' polygons.
Dwight and others apparently have had some success using the Onyx trees split into two parts.
Here's a thread about importing objects which may be useful -
http://archicad-talk.graphisoft.com/viewtopic.php?p=123343&highlight=tree#123343
I also believe people are bringing trees and entourage directly into Artlantis and C4D to render with even better results. I don't know for sure if the formats you mentioned are directly importable into the renderers, but I would have thought so.