Yes, Ginny Weasley is rumored to be cheating on Harry. Noooooo!
This is just gossip that has no real value because it's... well, gossip.
The word is that Bonnie Wright, who plays Ginny Weasley in the Harry Potter movies is now dating Jamie Campbell Bower who plays Caius in the upcoming New Moon movie. Caius is a bad guy Volturi who is happy to see beloved Bella good and dead.
They met because Bower is playing Gellert Grindelwald in HP and the Deathly Hallows which is currently filming. Grindelwald is Dumbledore's childhood friend who later becomes a dark wizard and Dumbledore's sworn enemy.
So there's your dose of useless rumor and innuendo for the day!
(I got the original scoop on this from ilovemovie's tweet. Check her out if you love movies.)