At version 2.7 we updated how default map navigation works when using an Apple trackpad or magic mouse. Details about this are here:
http://help.arcgis.com/en/webapi/javascript/arcgis/help/jshelp/intro_navigation.htm
I use a magic mouse with my mac so I haven't tested this behavior with a scroll wheel - if you press the shift button while using the scroll wheel does your map zoom in/out?