I have seen numerous of threads in which people have been asking question with regards to matching the Wattage & Ohm/s of the head/amp to the Wattage & Ohm/s the cab and if they can run the two ratings differ.
The following are some articles/threads should hopefully help.
http://www.talkbass.com/forum/showthread.php?t=144244
http://www.talkbass.com/ampfaq/
Update: 28 February 2006
http://www.duncanamps.com/technical/technical.html (Impedence calculator for your cab and other information relating to amps/heads)
Update: 04 April 2006
http://sound.westhost.com/impedanc.htm
(Impedance and Audio Equipment)
http://www.maxim-ic.com/appnotes.cfm/appnote_number/742 (Impedance: The scientific explaination)