I just recently (3 weeks ago) went through this.

And no, it wasn't for a vacation for something, the cats were moving 1600 miles to live in another state.

Things I learned (this was my first time traveling in the cabin w/ an animal):

1. You definitely need to reserve your space w/ Delta by calling in advance to make a note in your file. The check-in woman the day we checked in (you can't do kiosk or online check-in if you're traveling with pets) told me that the day before she had a guy try to travel with his small dog, but that they can't let someone do day-of pet travel. I don't know if she was confused, this is a new rule, or what, but I was very glad to have called in advance.

2. Compass, ASA, Mesaba, et. al. (all of the regional jets operating under Delta) only allow up to 2 pets per flight. This is why it's vital you call in advance to reserve your space if you're connecting/flying a regional jet. We were on a regional jet to ATL for our first leg.

3. The fee is non-negotiable, at least in my experience.

4. If you are Elite, yes, you can still be upgraded while traveling with your animal. My cat happened to enjoy her complimentary upgrade to first class very much

5. Make sure you have an "airline approved" soft-sided carrier (we used the Sherpa carriers, available at Petsmart, http://www.sherpapet.com/pet_carriers).

6. Many states have entry veterinary requirements for pets. To make sure we were covered, I had both of the cats examined by their vet, who filled out a Certificate of Health, certifying their rabies vaccination, et. cet. Nobody in the airport asked for this information, on departure or landing, but I wanted to be on the safe side since technically our new state did have these requirements (and some govt. officials like to be particularly picky -- not that I can blame them). I do know that if you send an animal by cargo the airlines mandate this form before they will accept your pet.

7. Be prepared for the unexpected. Case in point: I wasn't entirely aware that I would have to take my cat out of her carrier, put the carrier in the screening x-ray line, hold her in line for several minutes, and then walk her through the TSA screening area / human X-ray thing. Of course it makes sense, I just figured they'd hand wand the container or something. Good thing I was able to hold her long enough -- some friends' cats I know would have been sprinting towards the Hudson News Stand by that point.

8. Unfortunately, pets can't earn frequent flyer miles. I mean, infants-in-arms can, but kitties-under-seats can't? I digress.