Sonnet has created a dongle that allows you to use FireWire 400 devices in your MacBook Pro's FireWire 800 port. The Sonnet Simply Fast dongle works with standard 6-pin male FW400 cables.
This dongle can be bought from the Sonnet online store, or from other retailers (i.e. Amazon) for around $14.24US. You can visit the Sonnet website for more information about the FireWire adapter. If you are looking for solutions to this FW 400/800 mystery then you might also want to check out FireWire 800 to 400 cables which are very plentiful, and might be slightly cheaper.