Here's a jsfiddle example of how to do this.
var map = new L.Map('amap', { center: new L.LatLng(45.50144, -122.67599), zoom: 4, minZoom: 0, maxZoom: 18, layers: [ L.tileLayer('http://{s}.tile.cloudmade.com/{key}/997/256/{z}/{x}/{y}.png', { maxZoom: 13, minZoom: 0, attribution: 'Map data © 2011 OpenStreetMap contributors, Imagery © 2011 CloudMade', key: 'BC9A493B41014CAABB98F0471D759707' }), L.tileLayer('http://server.arcgisonline.com/ArcGIS/' + 'rest/services/World_Imagery/MapServer/tile/{z}/{y}/{x}', { minZoom: 14, maxZoom: 18, attribution: 'Tiles © Esri — ' + 'Source: Esri, i-cubed, USDA, USGS, AEX, GeoEye, Getmapping, ' + 'Aerogrid, IGN, IGP, UPR-EGP, and the GIS User Community'}) ]});
source share