At unicode.org the only locales listed as using Letter are the US and Canada.
Wikipedia agrees that only the US and Canada officially don’t use A4, but claims that in Mexico, Colombia, Chile and the Philippines Letter is nevertheless commonly used. Meanwhile at Microsoft the list of Letter using nations is: The US, Canada, Argentina, Brazil, Chile, Mexico, Venezuela and “Latin American countries”
As I’ve tried to document here, currently GTK thinks that the US, Canada and Puerto Rico are Letter users, while OpenOffice.org thinks that the list is the US, Canada, Mexico and Venezuela.
So, I still wonder, which nations excluding the US and Canada, would one truly find Letter as the true default in-use paper size.