In 2009, I decided to carry a pint of ice cream with me on a flight from San Francisco to Boston. When doing preliminary research, I immediately came across this Metafilter page where people described having successfully transported ice cream on planes.
A now-hard-to-find FAA PDF pointed at 49 CFR 175.10(a) in the DoT's hazmat regulations, which it summarized as permitting "small amounts of dry ice in carry-on (2 kg.) or checked baggage (2.3 kg.) in a package that allows venting of carbon dioxide gas." I called Virgin America and confirmed that they permit 2/2.3 kilograms of dry ice on flights.
I have a nice, insulated-but-not-airtight Craftsman padded lunch container that seemed about the right size, so I decided to go for it. I picked up ice cream a few days before the flight, stuck it in my freezer, and turned the refrigerator up to its maximum setting with the hope that the ice cream would be hard by the time of the flight (my fridge at the time was kinda old).
The morning of my departure, I made a few phone calls to nearby Safeway and Walgreens stores to see if they carry dry ice. They don't. I biked out to San Francisco Ice Company in Hunter's Point with my (small) messenger bag and managed to carry back 10 pounds of dry ice (the minimum quantity that they sell). It was cheaper than I expected, only $9. It usually comes in a big block, but the guy there cut it into gold-ingot-type bars for me. I ended up with seven bars, each maybe eight inches long, two inches tall, and an inch thick, all wrapped in a sheet of butcher paper. I was worried that it'd be uncomfortable to have the (-110 degrees Fahrenheit!) ice resting against my back on the ride home, but the messenger bag (and the lunch bag, which I'd brought with me) provided plenty of insulation.
After I got home with the ice, I took it, the Craftsman bag, the carton of ice cream, a resealable plastic bag, and a pair of oven mitts out to my patio. I sealed the ice cream in the plastic bag (didn't want it to leak everywhere if it melted) and wedged it into the Craftsman bag with three of the seven bars. I zipped it closed and left it on my counter to make sure that the ice wouldn't sublimate more quickly than I expected.
After an hour, the bars appeared to still be the same size. I could see the gas escaping from the corner of the bag, though, so I figured the ice was losing enough mass that I could get away with adding another bar to the bag and still be under (or at least plausibly near) the carry-on limit. I rearranged things so the ice cream was lying on its side surrounded by the four bars. I zipped the bag closed again, taped a piece of paper reading < 2 KG DRY ICE to it, and put it and a pair of work gloves into my backpack. As preparation, I also printed the relevant page from the FAA PDF to bring along with me.
When I approached my gate's security checkpoint in the International Terminal at SFO (yes, this was a domestic flight), I took the Craftsman bag and the pair of gloves out of my backpack and told the man on the other side of the conveyer belt that the bag contained a pint of ice cream packed in dry ice. I had to repeat this a few times before he understood; when I started opening the bag and CO₂ wafted out, another guard moved towards me and told me to stop.
"What do you have in the bag with the ice?", the first guard asked.
He scoffed. "You can't bring that in. You can only use dry ice for medicine."
I took out the FAA printout, which I'd moved my pocket as preparation, and showed him that I was permitted to carry up to two kilograms of dry ice on board to protect "perishables". He glanced at it and told me that he'd need to get his supervisor. They waved me through the metal detector and x-rayed the rest of my carry-on luggage. On the other side, another guard diverted the bag with the ice cream and took it to the kiosk at the end of the conveyer belt. He started putting on latex gloves (just to inspect the bag, I was hoping), and I offered to let him borrow my heavier work gloves. He declined, and I opened the bag and showed him the dry ice and carton of ice cream. He made some calls and told me I could go after a few minutes. We chit-chatted; he seemed nice enough after getting the go-ahead to let me through. I was worried that someone would play the ice-cream-may-be-a-solid-now-but-it-could-become-a-potentially-explosive-liquid-in-the-blink-of-an-eye card, but luckily it didn't come up.
When I finally opened my backpack after arriving in (warm in the summer: pay attention, San Francisco) Cambridge, there was frost on the outside of the Craftsman bag and my sweatshirt was frozen to it. I put on the gloves and opened the bag up to find all four bars of dry ice intact (the two top ones were smaller but the bottom ones were nearly their initial size) and the ice cream still frozen solid. If anything, it was too hard to eat, ten full hours after I first took it out of the freezer. The lining on the inside of the bag was torn up a bit, presumably from direct contact with the ice — I think that covering the inside of the bag with the butcher paper might've prevented this.
So it wasn't that tough to transport ice cream coast-to-coast. Checking my backpack as luggage would probably have made things simpler, but I'm too impatient to wait for checked luggage at the end of a flight, and I saw some mentions online of a ~$50 "hazardous materials handling fee" and was worried I might need to pay extra if I declared the ice while checking it, and I didn't want my backpack to miss the flight if there were any problems due to the dry ice. It's also possible that I could have used blue ice packs instead of dry ice if I checked the bag; I'm not sure exactly how cold the plane's baggage area gets, but the display on the back of the seat in front of me said that the outside temperature was around -60 degrees Fahrenheit when we were at 37,000 feet.