Before I get flamed for claiming the impossible, this is how I achieved a goal that was entirely in AJAX, but I needed it to across different domains!
Some background: Some time ago I found, and implemented this awesome heatmap click tracking for the company I work for. However, since our test machines run everything from one machine (i.e. images and code), when it came to putting it live I never thought for a second it wouldn't work...which was the result.
It was because the page was being served from one domain, while the click tracking data was being sent to our 'static' (non-mod_perl) servers.
I'm posting this up because I think the heatmap is a superb tool for companies, in particular the designers and the marketing people.
Heatmaps will tell both groups whether a particular element is working to focus attention, or if in fact something else is drawing attention that they never thought of.
The way I got around this was to send the data via an 'iframe'.
The click tracking needs the server side to record the data, and doesn't require a response. So, although using AJAX to perform the click tracking (on mouse down from anywhere in the page), equally I can record a mouse down from my main page, but instead, I'm changing the 'src' attribute of an iframe which sends the co-ordinates to my static server.
So where he has used the following:
I am simply ditching the AJAX aspect of it and changing it to (assuming 'iframe' is a variable that I have already pointed to our iframe):
Here is a simple example of it working (note that Safari doesn't throw any errors on the cross domain button).
Then I would style the iframe to sit outside of the window:
position: absolute; left: -10000px
Hopefully this is some use to someone else out there.