Hi,
I think Drupal 7's core architecture supports SSO between multisites. Alt...

Hi,

I think Drupal 7's core architecture supports SSO between multisites. Although I think there is a disclaimer that the sites need to be under same domain. There are modules that provide that the widely known is the Bakery project.

In your case you have sites on different TLDs, you can look into Multisite login. I have used it on Drupal 6 and they have the dev available for Drupal 7. Give it a try and see if it helps.

Cheers,
Swarad