Thanks a lot Leon,
Very helpful info. I understand that there is no mutual text regarding the relationship of the permanent residency of these two countries. What I want to find out, I guess is, if there is a text in the Canadian immigration law that prevents somebody that is a permanent resident of another country (including the US), to obtain a Canadian permanent resident status (PR card). I would highly appreciate any info/links in this regard.
No, there is nothing that forbids you from acquiring another country PR or citizenship while being PR of Canada. It might only come up if for example you had not met the residency requirements for the Canadian PR and wanted to appeal for it based on your circumstances and they were to find out that you had used your time in another country to gain PR or citizenship there, that might not reflect well on your case.
To keep your Canada PR, you just have to meet the residency requirements and stay out of trouble with the law.