<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by aussieinsf:
... you should get an additional 25% on the entire mileage of the second segment.</font>
I'm not sure about the bonus applying to the whole segment. I recently completed the Plat Challenge with a BOS-LAX segment. I got the base miles (2611), my Gold bonus (653), and then an entry that said "Platinum Challenge - G Points" (980).
This obviously did not add up to a 100% bonus for the whole flight. The Plat bonus apparently started at the imaginary point mid-flight where I completed the Challenge.