I don't know where else it's been published, but the best referenced statement
came in Lyle McDonald's book.

Susan

   It is the number of carbs average person needs to support brain. If
you go below 100g, body is forced to burn fat (either dietary or body),
produce ketones and goes into the ketosis...

Mirek

In general, if you eat 100g or more of carbs per day, that's enough to
prevent your liver from depleting its glycogen stores.  As long as
your liver has sufficient glycogen, it will assume it can burn carbs
(and thus store fat) for energy.

For many people, the key to losing weight is to switch the liver from
carb-burning to fat-burning (and thus slow down the insulin
production).  To do so, you must first deplete the liver glycogen and
keep it depleted.  The number of carbs per day to keep it depleted
varies from person to person, but the 100g number seems to be the
average.

Activity level is a big influence here; weight lifters can eat up to
50g carbs per workout extra for example.  Also, the brain will use
less carbs per day after you've adapted to low carbing, so you may
find that after a few weeks it's more difficult to stay in ketosis
(additionally, your body becomes more efficient and there's simply
less ketones to waste).

Another key is that the carbs per meal should be kept low (i.e. spread
your carbs throughout the day) to avoid insulin spikes.