Do all Italian surnames end in a vowel, and what is the most common Italian surname?

No, though many of them do. In the North-East of Italy, particularly in the region of Veneto, most surnames end in a consonant. (e.g. Marangon, Galan etc.)

[Jun 06 04 3:07 AM] vale70 writes:

The most common Italian surname is Rossi.
If you click on this link: http://gens.labo.net/it/cognomi/toptwenty.html you will see a chart (on the right) with the 20 most common Italian surnames.