Maggie Gyllenhaal bravely supported striking writers just recently with an admittedly fun faux-lesbian video saved by some self-mocking twists (excerpt below, whole thing here):
Britney Spears and Madonna, of course, kissed on stage at the MTV Video Music Awards December 2006.
Vanessa Hudgens just teased at it, lamely, last fall (still photo):
Television execs are not immune to the temptation; they've inserted girl-on-girl kisses into The OC, Wife Swa, Ally McBeal and One Tree Hill, and that was as of this three-year-old Times piece. Last year, it was Jennifer Aniston and Courtney Cox kissing on TV.
I'm already too bored to dig up more but if you're into it, go to town.