Where's Mike?

When NBC's Saturday Night Live opened its 18th season on Sept. 26, the biggest surprise was what viewers didn't see: Mike Myers, who is still filming his new comedy So I Married an Axe Murderer and is apparently in no rush to return. An NBC spokesman says Myers was granted permission to complete the movie, and ''may take time off'' after it's done, but will be back on SNL ''in calendar year 1992.'' A spokesman for Myers said the actor, who has three years remaining on his contract, would return to the show but didn't know when. Meanwhile, Saturday Night Live has taken note of his absence in a somewhat unusual way: Myers' name and face have been dropped from the opening credits.