In Ontario a solid yellow line, whether it's single or double, is a recommendation, not an absolute. The Highway Traffic Act spells out under what conditions you can drive "to left of centre"; essentially you need to be able to see what's coming. However, most people are under the impression that crossing the line IS illegal, so I very rarely do it.
My understanding is that all police forces will send out these "warning letters" to drivers based on complaints, but I assume they just have a form letter with a spot at the top to fill in the alleged offence - based on what the other driver reported, of course.