I need to find out what the ENTIRE url of the calling page is in VF, so that I can return to that URL.

I can't append the retUrl to the querystring on the button because &retUrl=%2F{object__c.Id} doesn't include the base url. 

I can't derive the retUrl from the object's record ID because I don't know the base URL and the base URL will change depending on circumstances.

I can't use ApexPages.currentPage().getHeaders().get('Referer') because that just returns "/". I thought maybe I was spelling "referer" wrong -- because it is actually spelled "referrer" in the HTML spec so I tried that, too, and it returned an empty string.

I can't use ApexPages.currentPage().getHeaders().getUrl() because that returns the CURRENT url.

I can't use ApexPages.currentPage().getHeaders().get('Host') because that returns the CURRENT host, and the current host is some weird VF thing instead of https://na*.salesforce.com.

And I can't use javascript (document.referrer) because that just returns an empty string, too.

I need a result that looks something like this: https://na6.salesforce.com/a0580000005XeTg

How can I do this?

TIA,

John