possible answer and more.
Page 142 in PHB, for Total Defense says can not combine Total Defense with Combat Expertise, or with fighting defensively.
Fighting Defensively as a full round action (page 143), You can choos to fight defensively when taking a full attack action.
Looking at page 92 for Combat Expertise the section for Normal kinda throws things off. But the Fighting Defensively does not say you can not use Combat Expertise.
I believe the intent of the rules is that with out Combat expertise, you can only get the -4 hit, +2 ac. I don't see anything saying you can not Fight Defensively and Take CE, The only thing that is a barring action is Total Defense.
FD, and CE together get you a nice +7 Dodge bonus to AC if needed, at a -4 hit. Not too bad. The Thead was one about making shields better, I don't know how to link them, sorry.
But I like the CE with small/large shield options givening a better return on CE.
I was thinking something like this
if you have CE
Exchange Rate is Buckler -1 hit/ +1 AC
Small Shield -1 hit/+1.5 AC rounded down
Large Shield -1 hit/+2 AC
or something like that.
http://www.enworld.org/showthread.php?t=174253 this is the older thread I think.