OK - you want side scrolling back for left and right? Go to the Trackpad settings, go to more gestures. On the "Swipe between pages" click on the drop down arrow and change it to the last option "Swipe with two or three fingers." You'll be able to swipe with 3 fingers between photos left and right. You'll also be able to swipe back and forth in web pages again. I don't know if it's a bug that two finger swiping sideways won't work or what. But that's what you do. There definitely appears to be a bug though in that if you change other options, it'll change that pages option back.
You da man. Turning on two or three finger swiping allows three finger swiping between photos in iPhoto.
Here's my theory re this problem. Two finger swiping is now that "gradual swiping" action you get in Safari. iPhoto is not equipped to do that, so it doesn't work there. Three finger swiping is the equivalent of old school two finger swiping, which is why iPhoto gets it.
Incidentally, if you set the Magic Mouse to do either one or two finger swiping, iPhoto also works with a two finger swipe that way. Nice to have that feature back, but then there's no swiping between full screen apps on the Magic Mouse.
Wonder whether (and if so, how) Apple with fix this.