It wasn't only cleared but suggested by George Lucas that Emperor Palpatine comes back, before that they were going to do Vader, but as I said, Lucas denied it and suggested Palpatine instead, which they immediately went with.
The rebirth of Darth Sidious is considered canon as it was cleared by George Lucas. It is in the realm of C-canon which is just below the movies and TCW. Anything in books or comics that doesn't infringe upon the movie canon is C-canon.
To put it simply, the rebirth is canon.