There doesn't seem to be a way to redeem them back to Bitcoins yet (that I've found). It may be best to contact BitBills and ask them there.
Bottom of the page:
http://bitbills.com/faqThanks but I did see that. I have contacted them earlier today but still no reply. Have I lost the BTC I bought if I can't redeem them? Has ANYONE redeemded their Bitbills for BTC?