Zio
Altough I didn't fully understand how the changing IP story made you a spammer in SORBS eyes, Italy is known for the big quantity of spam originating from computers located here, and for the lack of action about this from the tech guys of Telecom Italia.
At work we had a similar problem with some blacklist, and when we phoned them to ask what is happening, we found out that they had banned several C ip classes, because they were receiving spam reports for about two years now and still no response from Telecom Italia.
If you have a flat *DSL connection, buy an *DSL router and leave it always connected. This way you'll have the same IP for ever (except, off course, if the power goes down or something).
I saw that a lot more of italians are having the same problem, here on
Wikipedia.
If you need support in dealing with SORBS, we may help you if you post here their response.
Hope to help.
[later edit]
After checking the SORBS database, I found that my IP was in their list, too (they blacklisted the ips from 87.16.0.0 to 87.23.255.255, this means more than 320.000 IPs).
And guess what, the IPs are belonging to Telecom Italia
[/later edit]