I have a Mac too so that's why I thought of it.

There's not any mention of it on the Sony site that I've seen, but you can find several threads about it on the Adobe support forums.
I have read other posts in the past from people that they have not been able to transfer Overdrive library books from calibre to their 350/650's. I think it has something to do with the way the time expiration works. It seems that in previous posts most people recommend to use ADE to transfer library books. However, for a Mac user, ADE is not an option, and the Sony Reader software is what you should use.