To handle events on CSS circles properly, we have to ignore the corners of their bounding boxes. (found this copy/pasted on many blogs, thought I would keep here to clean up eventually)

circle_event_manager.js

$.CircleEventManager            = function( options, element ) {
    this.$el = $( element );
    this._init( options );
};

$.CircleEventManager.defaults   = {
    onMouseEnter    : function() { return false },
    onMouseLeave    : function() { return false },
    onClick         : function() { return false }
};

$.CircleEventManager.prototype  = {
    _init           : function( options ) {
        this.options = $.extend( true, {}, $.CircleEventManager.defaults, options );
        // set the default cursor on the element
        this.$el.css( 'cursor', 'default' );
        this._initEvents();

    },
    _initEvents     : function() {
       var _self   = this;
       this.$el.on({
           'mouseenter.circlemouse'    : function( event ) {
               var el  = $(event.target),
               circleWidth   = el.outerWidth( true ),
               circleHeight  = el.outerHeight( true ),
               circleLeft    = el.offset().left,
               circleTop     = el.offset().top,
               circlePos     = {
                       x     : circleLeft + circleWidth / 2,
                       y     : circleTop + circleHeight / 2,
                       radius: circleWidth / 2
                   };

                // save cursor type
                var cursor  = 'default';
                if( _self.$el.css('cursor') === 'pointer' || _self.$el.is('a') )
                    cursor = 'pointer';
                el.data( 'cursor', cursor );
                el.on( 'mousemove.circlemouse', function( event ) {
                var distance    = Math.sqrt( Math.pow( event.pageX - circlePos.x, 2 ) + Math.pow( event.pageY - circlePos.y, 2 ) );

                if( !Modernizr.borderradius ) {

                  // inside element / circle
                  el.css( 'cursor', el.data('cursor') ).data( 'inside', true );
                  _self.options.onMouseEnter( _self.$el );

                 }
                 else {

                   if( distance <= circlePos.radius && !el.data('inside') ) {                       // inside element / circle                       el.css( 'cursor', el.data('cursor') ).data( 'inside', true );                       _self.options.onMouseEnter( _self.$el );                                                   }                     else if( distance > circlePos.radius && el.data('inside') ) {

                      // inside element / outside circle
                      el.css( 'cursor', 'default' ).data( 'inside', false );
                      _self.options.onMouseLeave( _self.$el );
                    }
                   }
                }); 
            },
            'mouseleave.circlemouse'    : function( event ) {
              var el  = $(event.target);
              el.off('mousemove');
               if( el.data( 'inside' ) ) {
                  el.data( 'inside', false );
                  _self.options.onMouseLeave( _self.$el );
              }
             },
            'click.circlemouse'         : function( event ) {
              // allow the click only when inside the circle
                var el  = $(event.target);
                if( !el.data( 'inside' ) )
                    return false;
                else
                    _self.options.onClick( _self.$el );
            }
        });         
    },
    destroy             : function() {     
        this.$el.unbind('.circlemouse').removeData('inside').removeData('cursor'); 
    }
}