Post a screenshot comparison, I can't even remember the difference.
While the thinner font is cooler, the thicker font is much easier to read, IMO.
Apple must have received feedback that the lighter font was difficult to read and so they changed it. If they get feedback this is too thick maybe they'll add an option in the settings to switch between a normal and light font.
i doubt the change came as a result of feedback. I think what the final GM will have is ultra light beta 1/2 font for iPhones/ipods, and the regular beta 3 font for ipad's.
ios 7, is only ending up on retina display iphones, where the light font renders quite nicely. but its still coming to the non retina ipad 2 so they need to use the more legible on non-retina font there.