Well, I took them out of the car so I could weigh them and see if any had been venting. I found out I don't have a scale that accurate.
I also found out that one of my two stinkers has a corroded terminal. There's sulfur dust all over it. The terminal and the connector were sulfated together; I had to hit it with a hammer to get it loose, and when I did, I stripped the outer layer of the terminal off. (Pictures posted 2008-07-02.)
So now I need to make a decision: I can bet the batteries have had such light use that they're close to new and replace the stinkers with new ones, or I can remove the two stinkers. My car will lose a little range (more than the 1/6th you'd expect, because they'll all be working harder, and when batteries work harder they work for less time), but I don't need much.
My controller will go down to as few as 8 batteries, but my DC/DC converter is skirting the lower end of its range at 10 batteries.