[eschwartz]

There's a notice on calibre2opds.com about that -- If you own the books, then there should be no copyright issues. Unless you are sharing that folder with the world.
Dropbox only warns you if you have ebooks in your dropbox that are in a shared folder, and someone complains to the Dropbox team, "Hey this guy is illegally distributing the book I wrote!" And that will only happen if 1) You make that folder public, and 2) You publicize the link, and people start noticing.
This is all speculation, both on my part and by whoever shared his story on the calibre2opds.com blog.
What I know is this:
I have lots of DRM'ed ebooks in my dropbox, but only I can access them, and nobody has complained.
If storing your calibre library isn't a problem, then using calibre2opds shouldn't be any worse.
Unless you are using the DeDrm plugins, your books should still be locked to a particular device, in which case you could blog the link and it still doesn't matter -- only you can use it, so the rights-holder won't care.
In any event, any copyright problems have nothing to do with either calibre or calibre2opds or Dropbox, but rather, it is a problem if you distribute copyrighted content.