The change was to add a diode wired from +5vdc to the reset pin, cathode to +5vdc. The prior auto-reset wiring design could cause an intermittent latch-up problem when auto-reset was activated. This 'latch-up' problem could effect most prior arduino boards, not just the Uno boards, but was a little more prevalent when the Uno R2 design added a pull-down resistor on the 8u2 reset signal line. We never did find out why they added that resistor in the R2 design. Didn't really matter because we could duplicate the symptom on boards prior to R2, just not as frequently. There was a long post on this forum about this problem and I spend some time with others helping to try and verify if it was a real problem or not. It was real but pretty difficult to demonstrate in a repeatable manner. The problem was that the capacitance coupled reset pulse was sometimes triggering the arduino reset function into activating into it's high-voltage programming mode by creating a +10vdc pulse to the reset pin, rather then just a standard reset. Again it was not a common fault that everyone would experience as a lot depended on if a load was also wired to some of the I/O pins, like a pull-up or pull-down resistor, I forget which and which pins were most likely to help trigger the internal chip conditions needed to reproduce to the symptom.
Again there was a long posting on the subject that took some time to verify/reproduce the problem and analyze what was the cause of the latch-up condition. It then took some time for the Arduino company to also study the issue and either acknowledge the problem and come up with a solution or ignore it as 'seldom' being a problem. To their credit they did implement the solution, the added diode. Long story short, I expect to see the new external diode clamp used on all future arduino boards that utilize a capacitance coupled auto-reset pulse design and use the AVR mega chips. And it's pretty simple for any user to retro fix the solution by wiring a diode between reset and +5vdc, either on the Arduino board itself or on a shield connected board or by just sticking diode leads in the the shield pins (or on the ICSP pins) on the arduino board.