Amazon's Kindle software version 2.3 increased battery life from 4 days to 7 days; quite an improvement. Only the Kindle 2(*) model using HSDPA saw this improvement, the Kindle DX uses an EVDO radio and still lists a 4 day battery life.
It seems likely that the Kindle 2 shipped with incomplete radio power management to meet its shipment deadline, and this update represents the completed work. Nonetheless its fun to instead contemplate the moment when some firmware engineer poring over register settings utters a prodigious "WTF!?!" upon finding something completely bogus. A few keystrokes later and voila, huge battery life improvements...
