Apple - Can I disable the Magic Mouse swipe to go back feature in Safari?

Look under "More Gestures" the "Mouse" System Preferences pane:

Set it to two fingers or disable it altogether.

BetterTouchTool is a free solution: it lets you set up multi-touch gestures differently between different applications. I've had no issues using it yet, although I've only been doing so for a few days and only on Lion. It has received a respectable bunch of positive mentions from places like TUAW, MacWorld, and Lifehacker, and the only less-than-great reviews I've been able to find for it seem to be along the general lines of "I wish it also brushed my teeth" and "I turned off $feature and $feature stopped working"

Personally, it has saved my MagicMouse from the ignominy of sharing duty and mousing space with a wired Mighty Mouse.