Such bad news to hear today.
Giger's work on the ALIEN movie, such as the 'derelict' craft and it's interiors, along with the 'spacejockey' set and adult alien design were what gave it it's memorably nightmarish feel. They still remain the highest benchmark for 'otherworldly' production design as far as I'm concerned, and I'll never forget the unsettling vibe of my first big-screen viewing of them.
Sadly, Ridley's PROMETHEUS was Giger-'lite' when it came to it's overall designs, and I'd really been hoping for a lot more of Giger's actual involvement on the forthcoming follow-up... Too bad, alas.
He was a true talent with a unique art style, and I just wish he'd been asked to get involved in a lot more movies during his lifetime.