From the Canadian Consulate in Los Angeles (in response to an email I sent to them):
Citizens and Permanent Residents of the USA
If you are either a Citizen or a Permanent Resident of the United
States, you do not require a passport or visa to enter Canada. However,
you will be required to show proof of your identity as well as proof of
your citizenship or permanent resident status.
Citizens of the USA: You must travel with government issued photo
identification to establish your identity, such as: a valid driver's
license or State I.D. card; and documents to establish your citizenship,
such as: a valid U.S. passport, an original birth certificate issued by
the State (copies and hospital certificates and copies are not
acceptable); Citizenship/ Naturalization Certificate.
NOTE: The following are not considered proof of citizenship: Voter
registration cards; Army discharge papers.
Permanent Residents of the USA: You must travel with proof of your
Permanent Resident status, such as: your valid original I-551 Permanent
Resident Card/ Alien Resident Card/ "Greencard"; valid I-551 "Temporary
Evidence of Permanent Resident Status" endorsement in your passport; or
valid I-327 Permit to Re-Enter the United States. (Generally these
documents mentioned will satisfy both requirements to establish your
identity and your Permanent Resident status. However, if a clear
identification cannot be made from the photograph on your document, i.e.
if your photograph was taken when you were a child, you are strongly
encouraged to obtain a new document from the USCIS before travelling.)
Although it is rare, Immigration Officials may prevent the entry of
visitors:
who appear to pose a health risk to the Canadian public;
who are unable to demonstrate the ability to support themselves and
their dependants if granted entry to Canada;
who are unable to demonstrate their ability to leave Canada, or whose
willingness to leave Canada is in doubt if they were granted entry to
Canada.
Those admissible to Canada must not have a criminal record. (This
includes convictions for driving while intoxicated.) If you have been
convicted of an offence, you may be Criminally Inadmissible.
PLEASE NOTE:
A new United States' law requires all travellers, including US citizens,
entering or returning to the U.S. by air to have a valid passport
effective 23 January, 2007. Though Canadian entry requirements have not
changed all persons travelling by air from Canada to the U.S., will be
required to have a passport.
Similar requirements under the Western Hemisphere Travel Initiative will
apply to all travellers entering the U.S. by sea and through land border
crossings as early as January, 2008. An alternative travel document (a
passport card) is being developed by the U.S. government for US citizens
and is expected to be available before the end of 2007.
Persons travelling by air and returning to the U.S. prior to 23 January,
2007, will continue to be allowed entry upon presentation of
satisfactory evidence of identity and citizenship. Similarly, no
documentation change is foreseen for persons entering or returning to
the U.S. by sea or by land before January, 2008.
These new requirements do not apply to legal permanent residents (LPR)
of the United States. LPRs will continue to be able to use their Alien
Registration Card (Form I-551), issued by DHS, or other valid evidence
of permanent residence status to apply for entry to the United States.
Children of LPRs who are US citizens, however, will be subject to the
new passport requirements
Immigration Section
Canadian Consulate General
550 S Hope Street, 9th Floor
Los Angeles, CA 90071
Fax: 213 625 7154
http://www.losangeles.gc.ca
