I've no idea how large your forum is or how important the passwords are to you. But it may be possible that a smf->xf may emerge at some point. My wild guess would be that the passwords may be preserved.
Or if there is a ipb converter made available first, you could rent a 'hosted community' from them then import into that, so it could work like this smf -> ipb -> xf. The downside to doing that is that the 'permissions' will not carry over - but that is really easy to deal with compared to having thousands of peoples passwords changing.