Hi all
We posted a small tutorial on using Paypal, ActiveMerchant and Rails
in case you are not from the U.S 
(that means no authorized.net)
We only found scattered information here and there when trying to
locate some useful resources for it.
Hope you find it useful (of course any comments and recommendations
are welcome).
http://www.fortytwo.gr/blog/14/Using-Paypal-with-Rails
Regards
George