I dont think he's with Sienna. No one hung out and spent the night with someone of the opposite sex before without being intimate? the only reports come from the brittish press and their "sources". and even though this was before this weekend which seems to be their last because they wrapped or will these days, he kissed his girl and so far no too intimate photos of hayden and sienna. Swapping clothes is my second job so that doesn't really count for me but for some it may. And at the party she was singing with many others on stage and sat in someone's lap (I read this on imdb). If you believe gossip and speculation between two people who have so far shown they're close but nothing but friends then thats your choice. i just hate tabloids and they'll turn the story around all the time to make it seem like something that would sell.
So no, I most certainly won't believe he's with Sienna. Don't know who he's with but I don't buy the Sienna stuff until I see some actual proof.