Sending bitcoins to an un-owned, well-formed wallet
The wallet belongs to someone and you've just sent him money. (Can one
  contact this person and ask for it to be returned?) The key phrase is
  "can one contact" not "can you force him to give it back."


Not unless the owner of that address has publicly stated that she owns that address (perhaps on her blog or on a forum? Try googling).


  The wallet (as seen in the quote below) is unowned and is lost!?!


If the receiving address is valid and has not been used (you can find out from any online blockchain explorer), then it is highly likely that you've sent your coins to an address that nobody can claim (yet). In fact, it is almost certain that nobody will ever be able to generate a private key for that address. Therefore those coins will most likely be stuck in that address forever.

However, if the receiving address has been used, then there's a possibility of you getting in touch with that person and hope for her kindness. The problem is that there's no facility in Bitcoin for getting to know the identity behind a bitcoin address unless they explicitly provide this information.