gistfile1.js

var currentData = new Array();
var requestAgain = true;
var xhr = Titanium.Network.createHTTPClient();
var returnedData;
var timeDelay = (2 * 60 * 1000);  // 2 mins * 60 seconds * 1000 milliseconds

// Setup our event handlers
xhr.onload = function() {
  // This gets first one receipt of the data
  returnedData = this.responseText;
  // Let's turn the data from a string to a json dataset
  // We use another variable - to handle exceptions when parsing the data
  // so we don't lose the original data
  var rDataJSON = JSON.parse(returnedData);  
  // 
  // Ok - we have the data - we now need to do something with it
  processData(rDataJSON);
  requestAgain = true;
};

xhr.onerror = function(e) {
  Ti.API.info('GET returned error:' + e.error);
  // You need to decide here what to do - potentially xhr is in an invalid state
  // So it might need recreating
  // Let's stop the timeout from running
  clearInterval(timeoutHandler);
};

var timeoutHandler = setInterval(function() {
  // This where we do the request
  doAjaxRequest();
},timeDelay);

function doAjaxRequest() {
  // Only request again - if we have completed sucessfully before
  if (requestAgain) {
    requestAgain = false;

    // Ok - we should really be smart with this - 
    // we don't really want ALL the data returned everytime 
    // So you should talk with your server devs about passing a timestamp token 
    // which will allow the server only to respond with changed or new data
    // Returned data should have an ID or key we can use to selectively update our data

    xhr.open("GET","http://api.appcelerator.net/service?token=value");
    xhr.send();
  }
}

function processData(data) {
  // Process the data
  // I'm assuming that we have an array returned that has a string ID with each row
  // We don't handle deleating data :)
  for ( var i = 0; i < data.length; i++) {
	var thisItem = data[i];
	if (in_array(thisItem.ID, currentData)) {
		// The item already exists
		// You now need to decide what to do
		// Some Ti UI controls have an update method
	} else {
		// The item is new
		// You now need to decide what to do
		// Some Ti UI controls have an append method
		
	}
	currentData[thisItem.ID] = thisItem;
  }
}

// From: http://phpjs.org/functions/in_array:432
function in_array (needle, haystack, argStrict) {
    // Checks if the given value exists in the array  
    // 
    // version: 1006.1915
    // discuss at: http://phpjs.org/functions/in_array
    // +   original by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
    // +   improved by: vlado houba
    // +   input by: Billy
    // +   bugfixed by: Brett Zamir (http://brett-zamir.me)
    // *     example 1: in_array('van', ['Kevin', 'van', 'Zonneveld']);
    // *     returns 1: true
    // *     example 2: in_array('vlado', {0: 'Kevin', vlado: 'van', 1: 'Zonneveld'});
    // *     returns 2: false
    // *     example 3: in_array(1, ['1', '2', '3']);
    // *     returns 3: true
    // *     example 3: in_array(1, ['1', '2', '3'], false);
    // *     returns 3: true
    // *     example 4: in_array(1, ['1', '2', '3'], true);
    // *     returns 4: false
    var key = '', strict = !!argStrict;
 
    if (strict) {
        for (key in haystack) {
            if (haystack[key] === needle) {
                return true;
            }
        }
    } else {
        for (key in haystack) {
            if (haystack[key] == needle) {
                return true;
            }
        }
    }
 
    return false;
}

The send accepts parameters as defined here so you would need to provide a parameter or access a variable to determine what to send.

A specific example of posting something to twitpic can be seen here

If you're not seeing the "IN ONLOAD" in the log output then you'll need to reinit the xhr object - like this
You'll need to check the code for typos - I just changed in the gist - so something might have crept in.

You're not seeing any errors or warnings in the log output?