update marker location with leaflet API

The code inside your question is a little bit confusing, it’s hard to say what the issue is when you only post snippets.

As it is, this code:

    var lat = (e.latlng.lat);
    var lng = (e.latlng.lng);
    var newLatLng = new L.LatLng(lat, lng);
    marker.setLatLng(newLatLng); 

..should work as expected inside onLocationFound().

You can simplify it:

marker.setLatLng(e.latlng);

However, I guess the problem is a scope-issue, some of your variables (e.g. marker) is not accessible inside onLocationFound.

Here an example how to achieve it:

function init(){
    var map             = L.map('map', {center: [51.505, -0.09], zoom: 13}),
        marker          = L.marker(map.getCenter()).addTo(map),
        glcl            = google.loader.ClientLocation,
        onLocationfound = function(e){
          marker.setLatLng(e.latlng);
          map.setView(marker.getLatLng(),map.getZoom()); 
          alert('Marker has been set to position :'+marker.getLatLng().toString());
        };

    L.tileLayer('http://{s}.tile.cloudmade.com/[yourCloudmadeKey]/997/256/{z}/{x}/{y}.png').addTo(map);

    map.on('locationfound', onLocationfound);

    if(glcl){//when google.loader.ClientLocation contains result
       onLocationfound({latlng:[glcl.latitude,glcl.longitude]});
    }else{alert('google.loader.ClientLocation fails');}

    map.locate();
} 

Demo: http://jsfiddle.net/doktormolle/6ftGz/