A while back i did an experiment where i cut my daily intake to 1500 then 1200 calories. I did this by the standard dieters method of writing down everything i ate and looking up the calories in a table. I found that at 1500 calories i was able to do everything i do daily including biking a few miles to work but was hungry and losing weight. At 1200 calories, I was noticeable tired and couldn't think too clearly. I only kept this up for a few weeks with a few days at 1200 at the end.

I found this guy's website useful in understanding the carbo to body fat burning transition:
http://survival.tigerteam.se/food/
http://survival.tigerteam.se/excursions/0507-starvationstudy01/