Theoretically, if you have a refundable ticket, get bumped, and then don't take the new flight, you can get your original ticket refunded. However, the actual mechanics of the procedure could be very tricky. It will vary from airline to airline.

If you have an e-ticket, once you check in, the computer will show that the ticket has been used and you may have a problem getting a refund processed. If you have a paper ticket, some airlines collect the ticket itself when you check in and give you a boarding pass in exchange. You would need to retrieve the original ticket in order to get the refund. If the airline simply staples the boarding pass to the ticket and then collects the ticket when you board, you should be ok, since you still have the ticket. Then all you would need to do is submit the ticket for a refund.