I know I am preaching to the converted in this blog, but in case you need to remind someone else, here's the deal:
If you COULD be passed by someone on your right, whether in fact you actually ARE being passed or not, you are in violation of the Highway Traffic Act, section 147, subsection 1.
Paraphrasing, you MUST drive in the right-most lane unless passing or preparing to turn left,
If you ARE in fact being passed on your right, you are in violation of the HTA on two counts - 147 (1) above, AND Section 148 subsection 2, which reads (and this one is too cute not to quote in its entirety):
"Every person in charge of a vehicle or on horseback on a highway who is overtaken by a vehicle or equestrian travelling at a greater speed shall turn out to the right and allow the overtaking vehicle or equestrian to pass."
So you equestrians out there on the 401, please take note.