Whats the most calories you have burned in a day?

Probably close to 17,000 in one day... Yes, it was a very long day!

Lucky no-one had any kryptonite with them that day.

There was a lot of Kryptonite... I just avoided it...

16.5 hours of swimming, biking, and running

About 2600, running a marathon. A good rule of thumb for running or walking is approx 100 cals/mil

2800 on a 11 mile training run ... and I sorely disagree with your estimate up there.
I wear a Body Media Fit and I burn about 1700 calories for an intense hill-filled 9 mile run ...

I've read the 100 calories per mile "rule of thumb" in several running books, but I would imagine that just like a lot of the fitness machines, that's a set amount for the "average" weight of 150 pounds...people who are heavier might burn a lot more or lighter people burn a lot less...

This is correct. There is no "rule of thumb" for calories per mile because it depends on (a) mass and (b) energy expenditure. A mile walking is NOT the same as a mile running, because it taps into deeper aerobic (and perhaps anaerobic) reserves.

As a 240-ish pound male, I still expend about 195-200 calories per mile jogged (about 120 per mile walked). Obviously this will decrease a little as I get lighter, but it will also allow me to run faster and burn more in the same amount of time.

As to the original question, I have burned 2400 in a day. A combination of running and mowing the lawn.

1410 on May 2nd, don't remember why anymore, but recently have a few days on the 1000+ range.

Now that you mention it, its time to set it as a goal and go for 1500 in one day... next weekend that will be the goal!!!

I had already planned to walk/jog for 15Km so that will probably help.