Rachel McAdams changed into a semi-sheer Marchesa gown for the premiere of her new Woody Allen movie during the Cannes Film Festival this evening. She and boyfriend Michael Sheen were also together this morning at the Midnight in Paris photocall, though they seemed to show even more PDA at tonight's event — Michael sweetly helped with her dress as she ascended the stairs, and they huddled close while posing alongside costars Owen Wilson, Adrien Brody, and the French actress Lea Seydoux. Rachel and Michael made the trip to France together in order to promote the comedy after a stay in LA doing press and hanging out with his 12-year-old daughter, Lily.

Gwyneth Paltrow posed backstage at today's National Movie Awards in London. She stepped out for the event in a super low-cut black dress and nude heels after covering up in a hot pink Prada at last year's show. Gwyneth was back out on the town following a fun girls' dinner with Gwen Stefani last night. The busy moms have apparently been spending time together with their families while Gwen and her crew are visiting the UK. Gwen could be a source of advice for Gwyneth, since the actress has been considering recording her own album or even singing on a track with British band JLS. Gwyneth might have been able to cross paths today with the JLS boys, since they were also on hand at the show to perform.

The Cannes Film Festival kicked off with a star-studded opening ceremony that had the red carpet full of famous faces in the French city today. Salma Hayek posed in a gorgeous Gucci Première gown with her husband, Francois-Henri Pinault, on her arm, while model Karolina Kurkova stopped for the cameras solo in a blue Armani Prive. Uma Thurman, in a white Versace number, was accompanied by fellow judges Robert De Niro and Jude Law as they made their way up the grand staircase. Longtime couple Melanie Griffith and Antonio Banderas were also on hand for the start of the festivities, as was international actress Aishwarya Rai Bachchan, who wore Elie Saab. They all made their way inside the Palais des Festivals, where Uma, Jude, and Robert took the stage while Woody Allen and more watched from the audience. The event was just the first of many that will take place in the next week, with expected attendees Brad Pitt, Angelina Jolie, Johnny Depp, Penelope Cruz, and Sean Penn all making their way to the luxurious South of France to join in the fun.

Gisele Bundchen dressed up for a good cause in NYC last night in order to attend the United Nations For the Environment Programme’s Champions of the Earth Awards at the American Museum of Natural History. She's a goodwill ambassador for the organization and shared the stage with luminaries like Mexican President Felipe Calderón. It was the supermodel's third night of philanthropy in less than a week, after she attended Friday's Global Environmental Citizen Awards and Monday's Robin Hood Gala. She had her husband, Tom Brady, and his hair on hand for the latter event, which raised $47 million to fight poverty. Gisele's latest evening of charity was to kick off a UN-sponsored challenge for World Environment Day, which is on June 5. She and actor Don Cheadle are challenging fans to see which star can get more people to help plant a new forest — check out the World Environment Day page to get involved.