Add a note to tabs.onCreated documentation that the url might not be defined....

chrome/common/extensions/api/extension_api.json

 [
   {
     "namespace": "extension",
     "unprivileged": true,
     "types": [
       {
         "id": "MessageSender",
         "type": "object",
         "description": "An object containing information about the script context that sent a message or request.",
         "properties": {
@@ skipping 1300 matching lines @@
             "description": "Called when all the CSS has been inserted.",
             "parameters": []
           }
         ]
       }
     ],
     "events": [
       {
         "name": "onCreated",
         "type": "function",
-        "description": "Fires when a tab is created.",
+        "description": "Fires when a tab is created. Note that the tab's url may not be set at the time this event fires, but you can listen to onUpdated events to be notified when an url is set.",

[kathyw, 2011/01/07 20:06:14]

A couple of nitpicky comments:

url -> URL (x2)
an [url] -> a ...

(We've used "a" elsewhere, and we try to keep URL uppercase.)

         "parameters": [
           {
             "$ref": "Tab",
             "name": "tab",
             "description": "Details of the tab that was created."
           }
         ]
       },
       {
         "name": "onUpdated",
@@ skipping 3135 matching lines @@
                   "token": { "type": "string", "optional": true }
                 }
               }
             ]
           }
         ]
       }
     ]
   }
 ]