MapBox has created a CustomMarker plugin for Leaflet that should do the trick.
And another great example from Mapbox, GeoJSON Custom Markers and Style
Here's some sample code from that site:
var geoJsonData = {
"type": "FeatureCollection",
"features": [{
"type": "Feature",
"properties": {
"fillColor": "#eeffee",
"fillOpacity": 0.8
},
"geometry": {
"type": "Polygon",
"coordinates": [
[
[119.2895599, 21.718679],
[119.2895599, 25.373809],
[122.61840, 25.37380917],
[122.61840, 21.71867980],
[119.2895599, 21.718679]
]
]
}
}, {
"type": "Feature",
"properties": {
"marker-color": "#00ff00"
},
"geometry": {
"type": "Point",
"coordinates": [120.89355, 23.68477]
}
}]
};
var geoJson = L.geoJson(geoJsonData, {
pointToLayer: L.mapbox.marker.style,
style: function(feature) { return feature.properties; }
}).addTo(map);
NOTE: This is not part of the core LeafletJS library, it requires mapbox.js (and mapbox.css)