CHRONIC CONSTIPATION
I'd be the first to admit that constipation is pretty darn funny as long as it's not happening to you. This situation, however, was frustrating, depressing, and stressful before we got it under control.
Warning: I'll be talking about poop.
At the age of about three months, Flynn's activity level decreased and she started having a hard time defecating. We were surprised, since constipation is not a really common problem in kittens that age, but we tried the usual remedies: adding more canned food and some dairy products to her diet and mixing Benefiber in with her food. None of that worked. When she wasn't trying to go to the bathroom, she spent most of her time just sitting, her paws tucked under her, looking uncomfortable and depressed. She also quit gaining weight and wound up more than a pound lighter than her sister.
Having a sick kitten that you can't seem to help is really stressful, so off we went to the vet, who prescribed Lactulose, a stool softener. We faithfully gave her that for about a week and it did soften things up quite a bit, but she still couldn't go. She looked so uncomfortable one morning that we took her in for x-rays and an enema. The x-rays didn't show anything (except lots of poop). The vet was a bit mystified about the cause of the problem, but speculated that she may have a congenital problem with nerve function to her colon. In late June of 2009, we started her on a second medication: Cisapride, which helps with gastrointestinal motility, and began feeding her Science Diet I/D (Intestinal Diet). 
The combination of Lactulose and Cisapride has helped immensely, thank goodness. She still has some difficulty even when given both medications three times a day, but it's nothing like it was before. After a couple of days of receiving both medications, she started playing like a normal kitten again. After a little over a week, she'd gained half a pound. She hates the Cisapride, unfortunately, but it's more than worth the apparently nasty taste. 

At this point, we have no idea whether this is a phase she's going through or whether this will be a permanent problem. I have some suspicions that she doesn't absorb nutrients as effectively as normal cats do, since she eats much more than her sister and is oddly desperate about food. Could be she's just catching up after being ill, however.

October 2009 Update

Flynn has seen three vets now, including an internal medicine specialist, and all of them agree that the source of her problems is most likely a lack of nerve function to her colon - something she was born with. It looks like this will be a permanent problem that will require her to be medicated three times a day for the rest of her life. Good thing she's extremely laid-back and doesn't mind being medicated...
We attempted to lower her dose a while back, but she immediately began having problems. As long as she's on Lactulose and Cisapride three times a day, you wouldn't know anything is wrong with her. We switched to triple-fish flavored Cisapride, which she likes much more than the chicken flavor. She still eats quite a bit more than a normal cat, so apparently that wasn't just a phase she was going through.

September 2010 Update
We've tried several times to lower her dose of medication, but each time, she started having problems almost immediately. On the medication, she does very well. She still eats more than a normal cat without becoming overweight. We've long since quit feeding Science Diet to her and everyone else and switched all of the kitties to a canned-food-only diet. No doubt the higher moisture content is good for her in particular.

We will probably try to lower her dose again every so often, but odds are she's on medication for life.