I've talked to enough people and fished enough to believe that the odds of a Nor-Cal record are considerably lower than a So-Cal record. For all the time I've spent on the water, the biggest bass I've seen in Nor-Cal was in the 18-19lb range. I have never seen anything I thought was 20+. On the other hand, many reliable sources in San Diego have sighted fish over 20lbs on many occasions at multiple lakes.

For more concrete evidence you can look at the top 25 list
http://espn.go.com/outdoors/bassmaster/s/f_fea_bass_top25list.html

Isabella has a couple fish over 18 and Lake Success kicked out that 19lb 1/2oz fish in 2001. Those lakes are more central California than northern though. On the other hand, San Diego and So-Cal lakes take up most of the list and most of the fish over 20lbs. That's pretty good evidence to me that the odds in Nor-Cal are lower.

I will say that every time I look at Paul Duclos' monster from Spring Lake it sure looks over 20lbs to me. Have a look at the pic on the bottom of this page in particular:
http://www.worldrecordbass.com/gallery.asp?page=2&sort=1&Gallery=4

A fish like that keeps the dream alive for me.

The world record bass when (and if :)) it is ever caught, will be a single fish, just one bass that for a thousand reasons outlived and outgrew all other bass. To try to make some difinitive statement about where a fish like that will come from is to assume you can understand the thousands of of reasons it would exist in the first place. It is that uncertainty, that set of incredibly long odds, that makes the dream of the world record such an awesome dream :)