Both MtGox and Coinbase offer these extremely easy to copy/paste JavaScript buttons to accept bitcoin payments. And they both offer callback mechanisms where they 'push' completed orders back to your site.
But both services seem to omit any documentation about how I can link a completed order 'push' on my callback URL back to the customer who is at my site. The push JSON data has a transaction ID, etc. But I don't see any way to get matching data on the site that my customer visited. Now, if the javascript running on my web site that exposed the button created some JavaScript variable I could look-up in order to find a transaction ID at the conclusion of the transaction so that I could match the customer with the completed order I get from Coinbase, that would be great. But I see no documentation to that effect.
How do folks do this?