http://code.google.com/apis/maps/documentation/javascript/examples/map-coordinates.html

gistfile1.coffee

TILE_SIZE = 256

lat  = 41.850033
lon  = -87.65005229999997
zoom = 10

numTiles = 1 << zoom

bound = (value, min, max) ->
	if min then return Math.max value, min
	if max then return Math.min value, max

degreesToRadians = (deg) ->
	deg * (Math.PI / 180)

class MercatorProjection
	constructor: ->
		this.pixelOriginX = this.pixelOriginY = TILE_SIZE / 2
		this.pixelsPerLonDegree = TILE_SIZE / 360
		this.pixelsPerLonRadian = TILE_SIZE / (2 * Math.PI)

	fromLatLngToPoint: (lat, lon) ->
		x    = this.pixelOriginX + lon * this.pixelsPerLonDegree
		sinY = bound Math.sin(degreesToRadians(lat)), -0.9999, 0.9999
		y    = this.pixelOriginY + 0.5 * Math.log((1 + sinY) / (1 - sinY)) * -this.pixelsPerLonRadian

		[ x, y ]
		

projection = new MercatorProjection
worldCoordinate = projection.fromLatLngToPoint lat, lon
pixelCoordinate = (c * numTiles for c in worldCoordinate)
tileCoordinate  = (Math.floor(c / TILE_SIZE) for c in pixelCoordinate)

Ported some code from here to coffeescript, don't ask me how it works!