On Fastmac.com Store you can find a slot-In Blueray drive for your MBP:
http://fastmac.com/slim_bluray.php
Otherwise you there are other ideas for this free space:
- solder a USB Hub into it
- or a usb powered GPS or UMTS stick.
- or an unit to take the remote into it - with a slide in technique
- Buy an mediabay for a Dell notebook (ebay) and put a hdd into it the connector that must fit is properly an MOLEX connector.
thats from me at this point