The Atmel application note about using the TWI (AVR315 dated 09/04)
states that the TWBR should be set to a minimum of 10 and the prescalar
should always be set to zero; thus disabling the prescalar. However the
data sheet for the mega2561 does not mention either of these
restrictions. Has the TWI changed since the issue of the application
note or is it simply an omission in the data sheet.

I can think of at least one good reason for requiring at least 10 CPU
clocks for each cycle of SCL, which is interrupt latency. Does anyone
else have any ideas?
-- 
John B