[paraschopra]

Actually, it wasn't a sarcasm. I do need cross-domain user tracking for perfectly legitmate reasons. Many of my users do A/B test where control is www.example.com and variation is www.example.org (for example) and the goal page lies on www.shoppingcart.com -- currently there is NO way I can track a user across these three domains (no, third party cookies don't work properly on Safari and Opera).

I am left to experiment with user fingerprinting md5(user-agent, http accept and screen resolution) but that is less than ideal situation.

[jdunck]

If you don't need 100% reach, have a look at xauth source: http://github.com/xauth/xauth

They use a central domain and window.postMessage to make a secure but shared storage.