This from the law firm Clifford Chance:
How are non-Italian registered aircraft affected by the Tax?
Since non-Italian registered aircraft were added by way of a last minute amendment to a set of rules originally provided solely for Italian registered private aircraft, the new legislation is not properly designed to cover non-Italian registered aircraft. Absent official clarifications, the same rules for Italian registered aircraft should apply. Hence, unless an exemption applies, aircraft stationed in Italy for more than 48 consecutive hours will be subject to the Tax by reference to the period from 6 December 2011 until expiry of the certificate of airworthiness at the rate of one twelfth of the amounts referred to the above table for each month of validity of the certificate.
So if you plan to visit Italy, it pays to do so in the month your CofA expires.
The tax does not apply to local microlights, because they are not on the Italian national register. The law does not mention ULM, which could mean that foreign microlights are also exempt.
AOPA Italy has asked the tax authorities to confirm that the tax does not apply to Permit to Fly aircraft, since the wording only refers to the validity of a CofA. Unfortunately, the authorities have 6 months to response, which could mean waiting until August.
In the meantime, confusion reigns supreme.