British citizens can enter Italy and stay as long as they like on production of a full passport. Similarly unrestricted access is granted to all EU nationals, whereas citizens of the United States, Canada, Australia and New Zealand are limited to stays of three months, though they, too, need only a valid passport. All other nationals should consult the relevant embassies about visa requirements.
Legally, you're required to register with the police within three days of entering Italy. This will be done for you if you're staying in a hotel (this is why you have to surrender your passport on arrival), but if you're on a self-catering trip you should register at the Questura (HQ of the state police). It used to be the case that nobody bothered too much about this formality, but in recent years the police have begun to be more pedantic with backpacking types in Venice. So if you think you might look like the sort of person a Venetian policeman might deem undesirable, get registered.
Italian embassies and consulates
Getaway, Level 45, 1 Macquarie Place, Sydney, NSW 2000 tel 02/9392 7900; 509 St Kilda Rd, Melbourne, VIC 3004 tel 03/9867 5744; 12 Grey St, Deakin, ACT 2600 tel 02/6273 3333.
38 Eaton Place, London SW1X 8AN tel 020/7235 9371; 32 Melville St, Edinburgh EH3 7HA tel 0131/226 3631; 111 Piccadilly, Manchester M1 2HY tel 0161/236 9024.
275 Slater St, Ottawa, Ontario K1P 5H9 tel 613/232-2401; 3489 Drummond St, Montréal, Quebec H3G 1X6 tel 514/849-8351; 136 Beverley St, Toronto tel 416/977-1566.
6365 Northumberland Rd, Dublin 4 tel 01/660 1744; 7 Richmond Park, Belfast tel 028/9066 8854.
34 Grant Rd, Thorndon, Wellington tel 04/499 4186.
690 Park Ave, New York tel 212/737-9100 or 439-8600; 12400 Wilshire Blvd, Suite 300, Los Angeles tel 310/820-0622; 1601 Fuller St NW, Washington DC tel 202/328-5500. Consulates also in Philadelphia, Houston, Detroit, Boston, New Orleans, Chicago, Atlanta, Miami and San Francisco.