-> Thank you for reply. Actually, I do use your session module from the
-> wiki :) I even have plans to integrate SQLObject in it.

Cool!  If you do, drop it on the wiki -- I'm sure other people would
be interested.

-> Interesting solutions, both have their advantages, have to think..
-> But I am afraid at this point I don't understand the technical part of
-> cross-site authentication: how the other site knows who I am? I mean,
-> I log in on site 1. Cookie is created.... I am welcome... now here is
-> a link that leads to another site. Probably, the link must contain
-> that unique (session?)ID, so the other site will automatically
-> identify me? Otherway, how can it know?

Yep.  I imagine a link like this:

Go to site B.

Then when site B is entered, it knows (e.g. from the referrer URL) that
you're coming from site A, and it knows that you're user $uuid.

I don't know of any way to use cookies to do this automatically;
they're stored by site hostname, I believe.  You have to set the
cookies individually on a per-site basis.

cheers,
--titus