I am currently implementing 2checkout ( authorize.net ) into my server, so when a user signs up a script on my site is called and creates the virtual site etc.

Works ok, however what I am wondering: what will happen after a month??? When 2checkout.com bills my client, will they call the script on my site again - so I can update my billing history database???

I have looked in the docs from their site but could not find any info.

Good Question,

INMO your script wont be called after they bill the credit card, because the information passed (cart_order_id) etc seems to give an impression that they call the script only once ie when the customer enters the credit card information and is billed for the first time. I recommend you contact them directly to get the right answer :)