Maybe a gift certificate for a local spa - she can get a facial or a manicure or something. Put it in a card with a message "some self-pampering for your 40th."
I am in the same boat! I have lots of ideas but they are quite well off and already have loads of stuff, so am still stuck... Things I had thought of though - theatre tokens, nice china/vase from John Lewis (she loves John Lewis), a rose bush, jewellery, clock/watch. Does she have any hobbies?
Thanks for your answer that girl, but I asked this question back in August last year for her 40th Birthday, it's now March '06, 7 months later!! It's a bit late for presents now!