It's kind of amazing that someone like Audrey Hepburn existed. I've only seen two of her films to-date: Roman Holiday and Breakfast at Tiffany's. Nevertheless, from what little I know about her, she is endlessly captivating. Regardless...
Roman Holiday reminds me quite a bit of The Little Mermaid. Not the Disney one, but the original Hans Christian Andersen one. Actually, maybe not even exactly that one, but one definitely a lot more faithful to it. When I'd originally watched it somewhere around the age of eight, I think the idea of the mermaid disappearing into sea foam broke my heart. I'd go so far as to say that it's colored my worldview from that point on.
I'm sure The Little Mermaid was about a lot of things, but what sticks out in my mind is cultural immobility. She gives up her voice (her ability to communicate words and precise ideas) to gain legs (the appearance of a human). It seems pretty blunt in implying the prince and mermaid's love, though earnest, is only superficial. In the end (of the movie that I recall), even though her sisters provide her a means to return as a mermaid by killing the prince, she chooses to dissolve into sea foam instead.
In Roman Holiday, Joe and Ann fall quickly and earnestly in love, despite being from different worlds. In the end, though in love, they realize that what they had was beautiful, but ephemeral; true but out of reach.
If the movie were made today, Ann and Joe would probably end up getting married, living happily ever after. It's not that it couldn't happen. I recall a Japanese princess (Princess Akiko I believe) married a commoner not too long ago. And it's good in that it shows that love can overcome any obstacle; that hard work and perseverance can let you accomplish anything... You can create a multi-billion dollar business, too, I just don't think it's a realistic expectation.