Dr. Ian MacDonald at Florida State University just provided us with this BP memo detailing the spill-rate calculations performed by BP. Click it to see the full-sized version. We guess this is how BP came up with their very low estimated leak rates of 42,000 gallons (1,000 barrels) and 210,000 gallons (5,000 barrels) per day. The official estimated leak rate now stands at 20,000-30,000 barrels per day (and possibly much higher).
Read more about the varying spill-rate estimates, and this memo, in a Washington Post article by Joel Achenbach.