What is a good idiom for deceptive smile?

Not an idiom but you can say a sinister smile.

Edit: maybe too simple but: a fake smile, or an empty smile

Edit 2: I just found this online at Yahoo! Answers; I quite like this one:

Pan-Am smile > It is named after the airline Pan American World Airways, whose flight attendants would always flash every jet-setter the same, perfunctory smile.

Other alternative terms for a "fake smile" are: Botox smile, a say-cheese smile!

A fake smile only involves the zygomatic major muscles which are forced by a person in order to give a polite smile. The smile is then limited only to the lips and there is no contraction in the eye area.

The study of smiles is a part of gelotology, psychology, and linguistics.

And it may be good to know that scientific word for a "real smile" is "Duchenne Smile" which is named after physician Guillaume Duchenne who first recognized the muscles involved with this smile.

It depends what you're trying to convey. If it's a lack of emotion, or a lack of interest, it could be "a pale smile", "a polite smile", "a perfunctory smile", "a mechanical smile".

To suggest that it's regularly used, "a practised smile", "a receptionist's smile", "a salesman's smile", "a politician's smile".

If there is any negative intent, it could be "an intimidating smile", "an icy smile", "a debt-collector's smile"...

Reading the comments and answers here I was reminded of an excerpt from Ralph Waldo Emerson's Self-Reliance:

There is a mortifying experience in particular which does not fail to wreak itself also in the general history; I mean "the foolish face of praise," the forced smile which we put on in company where we do not feel at ease in answer to conversation which does not interest us. The muscles, not spontaneously moved, but moved by a low usurping willfulness, grow tight about the outline of the face with the most disageeable sensation... [A] man must know how to estimate a sour face... If this aversation had its origin in contempt and resistance like his own, he might well go home with a sad countenance; but the sour faces of the multitude, like their sweet faces, have no deep cause, but are put on and off as the wind blows and a newspaper directs.

Whatever the context, I think forced smile works, because the smile is as disingenuous as the "crocodile tears." The tears aren't coming from real sadness; this smile isn't coming from true happiness. Regardless of motive, the smile is not a reflection of how the smiling person feels. (The references I've found, though, tend to associate "forced smile" with one done from a non-hostile attitude.)

If the context matches Emerson's scenario -- where the smile comes more from polite obligation than how its wearer feels -- I recommend strengthening the reference by using Emersionian, as with:

1. The Emersonian "'foolish face of praise'"
2. The Emersonian "forced smile"
3. The Emersonian "sour faces (of the multitude)"

Depending on the context -- and on whether it's an idiom for a deceptive (i.e. deceitful) smile or a fake smile that you're looking for -- you might want to consider the following phrases:

Affected smiles:

put-on grin, coated smile, made-up grin, plastered smile, placard smile, frontage smile, facade grin, storefront grin, meet & greet smile, front desk smile, greeter's smile, checkout smile, bogus grin, slick-perfect (or slick cover) grin, schoolmarmish/schoolma'amish grin, straitlaced smile, tight-laced smile, Pecksniffian smile, pharisaical smile, whited sepulcher-ish smile, holier-than-thou smile

At the sound of her name, my mother offered a put-on grin that looked as if she had found the missing half of Cheney's smile. source

There was a lot of anger in the house, but everybody kept it hidden behind a coated smile except Annie. source

Throughout the narrative, the patient wore a plastered smile. source

Stoop waiting home with frontage smile... source

I can see the sweet sweat on your forehead, and the quivering falter of your facade grin... source

No meet & greet smile and seems the employees are miserable also.source

A front desk smile might have been inviting! source

Florio turned around, his greeter's smile on his face... source

She hands me the bag and we smile a checkout smile. source

The moment Charles entered the room with that bogus grin on his face... source

Displaying a schoolmarmish grin albeit with an unconvincing LA inflection. source
Opposing both, she flashed a schoolmarmish grin... source

I ushered him in, a tight straitlaced smile as a greeting. source

A falsely tight-laced smile spread over Hogarth's face. source

He would stretch forth a finger and help him with a Pecksniffian smile. source

Outside of Chicago, we wear a pharisaical smile... source, source

A whited sepulchered smirk and oh so jaded. source, source

She's so manipulative, with that holier-than-thou smile. source source

Deceptive smiles:

Sleek smile, phony smile, humbugging grin, carpetbagging smile, shyster's grin, con artist grin, huckster's grin, two-timing smile, double-crossing smile, two-faced smile, double-dealing smile.

The notice calling the meeting of protest gives a curious insight into the war of angry emotions that may be hidden under the sleek smile of the supplest waiter. source

She had confessed, instantly, with her humbugging grin... source

He still can't see the fat face hidden behind his glasses, behind a carpetbagging smile. source http://www.encyclopediaofarkansas.net/encyclopedia/entry-detail.aspx?entryID=1179

"You should just go back to show everybody in there at least that you're alright", he said while flashing a shyster's smile. source source

She has never been so relieved to see that totally untrustworthy honey, I'm hip huckster's grin on her husband's face... source source

Girls manipulate and bully almost silently, often behind a two-timing smile that may show intentions to their classmates. source

Smiling her double-crossing smile, Regina offers to introduce Sally to Aaron but smooches him instead. source

...at seeing baby's first smile -- and a dark side -- a two-faced smile in which the outward pleasantness camouflages a person's inner feelings. source source