Lip-Lock Sun-Block

The sun is setting. Two character gaze longingly into each other's eyes. The camera shifts to a profiled two shot, with the setting sun shimmering between them in the background. Their faces now silhouettes, the characters move closer, slowly covering the sun until at last... their lips meet.

D'awwww.

A fairly common kiss, combining the power of the sun with the Power of Love. Variations include kissing in front of the moon or some other light source, or holding each other without kissing. Expect lots and lots of Lens Flare.

Interesting note: From a lighting standpoint, when the actors are backlit their faces are obscured or silhouetted. Once they kiss, they block out the light coming through, meaning the camera can finally see their faces. A kiss turning shadows into people... There's something symbolic in that.

Compare Full-Moon Silhouette. A subtrope of The Big Damn Kiss.

Examples of Lip-Lock Sun-Block include:

Film

The Kiera Knightly movie version of Pride and Prejudice has one of these near the end, when Darcy and Lizzie confess their love. Of course, it's sunrise, not sunset, and there's technically no kiss, but the concept is there.
Slumdog Millionaire has one in the last few seconds of the film. Watch it here.
In True Lies, there's a variation involving Jamie Lee Curtis and Arnold Schwarzneggar. Instead of the sun, their kiss blocks out a nuclear explosion.
The Island Right before Lincoln Six Echo and Jordan Two Delta launch their rescue mission, they discuss what they'll do after, and then, Cue the Lip-Lock Sun-Block...

Live Action TV

Gwen and Arthur's first kiss in Merlin. Even those who dislike the ship are forced to admit that it's a beautiful, beautiful kiss.
In Pushing Daisies, Ned and Chuck do this in a flashback to when they were kids. It's adorable.
The Monkees episode "One Man Shy" has this trope (Peter and Girl of the Week, Valerie) during one of the romps to "I'm a Believer."
The Rose/Doctor kiss in the Doctor Who episode "The Parting of the Ways". Bonus points for having the freakin' time vortex providing the background light.