From Wiki

If you become a Canadian citizen, you may apply for a Canadian passport and use this to enter the United States instead of the Visa Waiver Program with a British passport.

[edit] What changes (as a tourist)

• You no longer need to fill in a green I94W form. At the border, this means that you usually do not have to leave your vehicle.

• You can be admitted for 180 days, rather than 90 days on the visa waiver program.

• You can fly into the USA on a private aircraft or enter on a yacht (this is not allowed on the Visa Waiver Program)

• You can travel into the USA by air or other commercial transport without a return/onward ticket.

• Canadian citizens entering the USA for tourist purposes are not fingerprinted under the US-VISIT program.

• On the land frontier, you may use an Enhanced Drivers Licence (not available in all Canadian provinces) in place of a passport.

[edit] What does not change

• You have no automatic right to be admitted to the USA.

• US Immigration can, and often does, refuse entry to Canadians who are criminally or otherwise inadmissible to the United States, or where they believe a Canadian citizen is not a genuine visitor.

• The allowable activities, in terms of tourist or short term business activties, are exactly the same as if you use the Visa Waiver Program or have a B1/B2 visa.

• You are still expected to leave at the end of your admission period.

[edit] Longer term entry to the USA

• If you want to spend time in the USA in another nonimmigrant category, such as H1B, L (inter company transferee) or F (study) you must meet exactly the same rules as anyone else.

• There is the TN status specifically for Canadian citizens.

• Most of the time, you can apply for H1B, L, F or TN status at the border or on arrival in the USA. You do not need a visa label in your passport (although any family members who are not Canadian citizens do)

• For certain categories of nonimmigrant admission, such as E (treaty trader/investor) or K (fiancee), Canadian citizens do need a visa obtained in advance.

• Canadian citizens in most non-tourist nonimmigrant categories, as well as those who are permanent US residents, are now included in US-VISIT fingerprinting.

• If you later on look to apply for a Green Card or Immigrant Visa (permanent US resident) then the rules are exactly the same as for anyone else.

• In fact, if you are applying for a Green Card, US Immigration will look at your birthplace for determining country specific quota. Not an issue for those born in the United Kingdom but it often affects those born in India or mainland China (unless such persons have a spouse or parents born elsewhere).