Access is denied when using SOAP through Javascript in CRM Online

I know this question has been asked a thousand and one times, however none of the proposed answers solve my issue.

I have followed the Microsoft SOAP walkthrough (http://msdn.microsoft.com/en-gb/library/gg594434.aspx), so I know the code is correct.

This is being run, like in the walkthrough, from a HTML webresource in the CRM context itself, so surely the authorisation is performed by the application itself?

The line the error occurs on is the .open method of the XMLHttpRequest, where I recieve an 'Access Denied' error with no further information

var xmlHttpRequest = new XMLHttpRequest();
xmlHttpRequest.open("POST","https://[my org].api.crm4.dynamics.com/XRMServices/2011/Organization.svc/web", true);

I thought SOAP within the CRM context would be easy, that showed me. If anyone has solved this issue, or can direct to someone who has, that'd be grand :)