Miley Cyrus Engaged to Liam Hemsworth -- Where There's Smoke ...

Miley Cyrus isn't Liam Hemsworth's only bad habit -- hours after news broke about the couple's engagement, the actor surfaced on a movie set ... lighting up a filthy cancer stick.

Liam chipped away at his life span during a quick break in New Orleans -- where he's currently filming "Empire State" -- and he wasn't eager to talk engagement.

The story broke today ... Liam and Miley are getting hitched after dating for three years.