Very little is publicly known about the CANUSA agreement. One thing we do know is that Canada-U.S. negotiations for the agreement were well underway by mid-1948. According to a U.S. Air Force memorandum dated 7 June 1948 that described a draft version of the agreement, it is modelled at least in part on the U.K.-U.S. BRUSA agreement (presumably the 1946 version) and governs Canada-U.S. co-operation on Communications Intelligence, which, for the agreement's purposes, is "understood to comprise all processes involved in the collection, production and dissemination of information derived from the communications of countries other than the U.S.A., the British Empire, and the British Commonwealth of Nations."
According to the memorandum, the agreement provides for the exchange of COMINT information "on the request of each authority to meet the requirements of the COMINT centers for assistance in the efficient discharge of their mutually agreed-upon COMINT activities and undertakings" and "on a 'need to know' basis as determined by the originating authority." It also provides for the exchange of COMINT liaison officers between Canada and the United States. [Brig. Gen. Walter Agee, USAF, Acting Deputy Director of Intelligence, "Memorandum for the Coordinator of Joint Operations: Proposed U.S.-Canadian Agreement," 7 June 1948.]
Sources disagree on when the CANUSA Agreement was finalized. Martin Rudner, one of the best informed commentators on CSE matters, says the agreement was finalized in May 1948. [Canada's Communications Security Establishment: From Cold War to globalisation, NPSIA Occasional Paper 22, 2000.] But this date seems incompatible with the June 1948 Agee memorandum. David Bercuson and Jack Granatstein put the date at some time in 1949. [D.J. Bercuson and J.L. Granatstein, The Dictionary of Canadian Military History, Oxford University Press, 1992, p. 43.] Others have suggested that the "Security Agreement between Canada and the United States of America" signed 15 September 1950 may have been the CANUSA agreement.
My own sense is that 1949 is about right. In fact, this declassified letter from the Chairman of the U.S. Communications Intelligence Board to the Chairman of the Communications Research Committee (the Canadian government interdepartmental committee then responsible for SIGINT policy), dated 29 June 1949, may be the actual document that finalized the agreement.
In any case, the Public Accounts for FY 1949-1950 show that Robert S. McLaren, Canada's first SIGINT liaison officer to the U.S., received his moving allowance and the initial installment of his representation allowance during that fiscal year (i.e., between 1 April 1949 and 31 March 1950). This also suggests that CANUSA was finalized in 1949 or very early in 1950.
Canada and the U.S. continue to post liaison officers at each other's agencies. Information about one former Senior U.S. Liaison Officer/Ottawa (SUSLO/O), Velva Klaessy, can be found on the NSA website.
Update 30 May 2009: According to Secret Sentry, the letter above is indeed the document that finalized the agreement. The agreement was formally signed in November 1949.
Update 23 April 2017: The 1949 version of the CANUSA Agreement, including Cabell's reply shown above, was released in April 2017.