The convention seems to be leaning toward latitude first followed by longitude.
In Google Maps & OpenStreetMaps as you scroll around it prints in the url latitude followed by longitude.
In Bing Maps & MapQuest if you right click somewhere it displays it’s latitude followed by longitude.
Bing and Google Geocoders&API both expect latitude followed longitude.
However I’m seeing some functions in Cartesian3.js that expect longitude, latitude order. Namely the ones that start with Cartesian3.fromDegrees and Cartesian3.fromRadians.
Also the constructor: var Cartographic = function(longitude, latitude, height) is longitude, latitude order, although being an object rather than an array the components in Cartographic don’t really have an order once created.
Granted it’s relatively simple to reverse the elements when using these functions, but it would be convenient if they instead used the prevalent convention. Not because the prevalent convention is better, but simply because it’s something people are familiar with. Otherwise the order would be inconsequential.