It seems that United is taking the easy way out saying we did all the routine maintenance but "stuff happens" and we couldn't know that it would (i.e. "extraordinary circumstances") but it is not technically supported by case law, ECJ case C-549/07. From what I understand enforcement of EC261 by EU community regulating bodies is kind of lax and individuals who have been denied compensation have to go to court. Here is some information that may help the OP with his/her appeal to the ENAC (Italian Civil Aviation Authority)

-all bolding is mine

http://passagerrettigheder.dk/EU_COU..._December_2008

The above link provides some rulings by the European Court Justice (ECJ) judgments on regulation no. 261/2004.

Article 5(3) of Regulation No 261/2004 contains the Concept of "extraordinary circumstances" which tends to give airlines a "Get Out of Jail Free card"; however ECJ court rulings tend to requires airlines to show more than "adhering to maintenance schedule" as a demonstration that the mechanical cause was an "extraordinary circumstance".

From Friederike Wallentin-Hermann v Alitalia – Linee Aeree Italiane SpA (22. December 2008 case C-549/07)http://curia.europa.eu/jcms/jcms/j_6/
Air carriers are confronted as a matter of course in the exercise of their activity with various technical problems to which the operation of those aircraft inevitably gives rise. The resolution of a technical problem caused by failure to maintain an aircraft must therefore be regarded as inherent in the normal exercise of an air carrier’s activity.

Operative part of the judgment
Article 5(3) of Regulation (EC) No 261/2004 of the European Parliament and of the Council of 11 February 2004 establishing common rules on compensation and assistance to passengers in the event of denied boarding and of cancellation or long delay of flights, and repealing Regulation (EEC) No 295/91, must be interpreted as meaning that a technical problem in an aircraft which leads to the cancellation of a flight is not covered by the concept of 'extraordinary circumstances' within the meaning of that provision, unless that problem stems from events which, by their nature or origin, are not inherent in the normal exercise of the activity of the air carrier concerned and are beyond its actual control. The Convention for the Unification of Certain Rules for International Carriage by Air, concluded in Montreal on 28 May 1999, is not decisive for the interpretation of the grounds of exemption under Article 5(3) of Regulation No 261/2004.

The frequency of the technical problems experienced by an air carrier is not in itself a factor from which the presence or absence of 'extraordinary circumstances' within the meaning of Article 5(3) of Regulation No 261/2004 can be concluded.

The fact that an air carrier has complied with the minimum rules on maintenance of an aircraft cannot in itself suffice to establish that that carrier has taken 'all reasonable measures' within the meaning of Article 5(3) of Regulation No 261/2004 and, therefore, to relieve that carrier of its obligation to pay compensation provided for by Articles 5(1)(c) and 7(1) of that regulation.

In this judgement the court provided the following discussion:
23 Although the Community legislature included in that list ‘unexpected flight safety shortcomings’ and although a technical problem in an aircraft may be amongst such shortcomings, the fact remains that the circumstances surrounding such an event can be characterised as ‘extraordinary’ within the meaning of Article 5(3) of Regulation No 261/2004 only if they relate to an event which, like those listed in recital 14 in the preamble to that regulation, is not inherent in the normal exercise of the activity of the air carrier concerned and is beyond the actual control of that carrier on account of its nature or origin.

24 In the light of the specific conditions in which carriage by air takes place and the degree of technological sophistication of aircraft, it must be stated that air carriers are confronted as a matter of course in the exercise of their activity with various technical problems to which the operation of those aircraft inevitably gives rise. It is moreover in order to avoid such problems and to take precautions against incidents compromising flight safety that those aircraft are subject to regular checks which are particularly strict, and which are part and parcel of the standard operating conditions of air transport undertakings. The resolution of a technical problem caused by failure to maintain an aircraft must therefore be regarded as inherent in the normal exercise of an air carrier’s activity.

25 Consequently, technical problems which come to light during maintenance of aircraft or on account of failure to carry out such maintenance cannot constitute, in themselves, ‘extraordinary circumstances’ under Article 5(3) of Regulation No 261/2004.

26 However, it cannot be ruled out that technical problems are covered by those exceptional circumstances to the extent that they stem from events which are not inherent in the normal exercise of the activity of the air carrier concerned and are beyond its actual control. That would be the case, for example, in the situation where it was revealed by the manufacturer of the aircraft comprising the fleet of the air carrier concerned, or by a competent authority, that those aircraft, although already in service, are affected by a hidden manufacturing defect which impinges on flight safety. The same would hold for damage to aircraft caused by acts of sabotage or terrorism.

27 It is therefore for the referring court to ascertain whether the technical problems cited by the air carrier involved in the case in the main proceedings stemmed from events which are not inherent in the normal exercise of the activity of the air carrier concerned and were beyond its actual control.