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.
Only in the EU is it canon. He is killed and gone forever in the film universe.
"Once Vader dies, he doesn't come back to life, the Emperor doesn't get cloned and Luke doesn't get married." - Lucas, 2008