that's the question isn't it? 
I know there's an importer for 3ds max. It extracts the data from SL (to get around the issue of the different file layouts I assume)and imports but there are no importers for any other 3d tools that I can find so I assume that the demand hasn't been there previously as (I assume) moving the files has been confined to SL compatible platforms.
However, Blue Mars is now on the scene. I confess I really, really don't want to have to start again and would love to be able to lever something out of all my SL builds - and I suspect a lot of SL creators will feel the same way, but it's too early to tell yet.
There's already an exporter built into one of the SL clients (emerald) so making it compatible with those would be great as it would put a hurdle before those inworld who would seek to download content that they shouldn't be.
/me smiles winningly and looks hopeful