Uma Thurman Engaged?

Actress Uma Thurman is reportedly engaged to boyfriend Arpad Busson.

The 'Be Cool' star -- who has been dating Swiss financier Arpad since last year -- showed off a diamond ring so big "she can't fit it through the sleeve of her coat" at Elton John's 10th White Tie and Tiara benefit in Windsor, England, last night.

An onlooker tells America's New York Daily News newspaper, "All the ladies' eyes were buggin' out because the ring was so massive! It was so huge that she couldn't fit it through the sleeve of her coat."

Thurman -- who has two children with ex-husband Ethan Hawke -- was shocked when Arpad presented her with the ring during their UK trip, which will also see them celebrate former South African president Nelson Mandela's 90th birthday.

A source close to the actress reveals, "It was a really old-fashioned romantic gesture.

"Everyone is so happy for Uma after what she went through with that stalker," added the pal, referring to Thurman's recent court ordeal with obsessed fan Jack Jordan. "Arki is a guy who can take care of her.

"Uma is hoping this time, it's really love," said the friend. "Everyone wants the best for her. And Arki is a really sweet guy."

Busson, born in 1963 in Switzerland, heads the EIM investment company, which manages assets of over $13 billion, according to the company.

Uma was previously married to actor Gary Oldman, while Arpad has two children with Australian supermodel Elle Macpherson.