My grandparents were married in Toronto on June 1, 1911, the same day as the census. I cannot find them listed anywhere, and have just checked the new version of the 1911 census. I've even checked page by page hotel census' in Niagara Falls in case they went there on a honeymoon! Perhaps they just missed it altogether and didn't bother. Any advice welcome.

June 1, 1911 is the official date of the census - the day enumeration began and the date each enumerator was told to use in questioning (e.g. As of June 1st how old were you?).  The enumeration process actually took several months.

If a person is missing from the census there could be several explanations besides them not being home.  They could be mis-indexed under names that would not be obvious or they could have moved before enumeration took place in their area to an area where it had already been completed.

 

Check the Toronto City directories for 1911-1912 to get the address where they lived.  Then use that address to locate who was enumerated there in the 1911 census.  

It's also possible they were enumerated as single or they were living in someone else's household and were mis-indexed under that surname - check their relatives or do a search without surnames.