I'm great at burning fat throughout the rest of my body, but the pecs and abs are especially difficult. What are the best exercises to burn fat about the mid-section? Are situps or other muscle-building exercises appropriate, or will that just build muscle?

Hard work is all you can do really.

Here is a site with a lot of good advice.

Actually, cardio exercises are really the best exercises to do to lose weight anywhere on the body, such as running, riding a bike, swimming, etc..Although, I'm a big stickler for running, I don't like doing it, but I think it has the best effect on weight loss b/c you're working extra hard, you'd have to swim or ride a bike for a lot longer than just running for a mile or two. Just doing crunches and such just works the muscles and builds the ab muscle, it doesn't help shed many pounds. Wish I could follow my own advice!

okay, i'm assuming you're not familiar with abdominal exercises...

crunches will help, yes, but only for the top of your "tummy". it's great if you can incorporate it with other abdominal exercises, though.

whenever doing abdominal exercises, ALWAYS breathe out, and contract your stomach when you do the exercise. try to begin and maintain a breathing rythym.
NEVER BREATHE IN!!! if you do, you'll just end up with a muscular potbelly. and wouldn't that be lovely.

i find that doing leg lifts or holding my legs up at a 40 egree angle (just shy of half of straight up and down, for those incaapable of math, such as moi) and gently rapping/pounding on my stomach so i don't feel that annoying burn and feel like stopping.
also, push ups, just plain push ups, are good for your shoulder area (pects, deltoids, and upper biceps/triceps).

ahhh, now doesn't that feel better?

To expand on Sherasi & SweetnSour's correct answers; there is really nothing you can do to direct "fat burning" to any portion of your body.

Fat is burned according to pre-programmed tendancies in each person, generally in the reverse order in which it was gained. Doing abdominal excercises may increase the muscle mass of your abdominals thereby reducing the proportion of fat to muscle mass.

If your abdominals have a history of being "fat" it's likely that you'll have to do some rather extreme calorie control to get rid of the fat. Also, the mid section is often a prime location of fat storage due to a condition known as "insulin resistance." That's way to complex to discuss here - but doing a Google search on the "insulin resistance" will provide you with tons of info.