Maybe Ridley's going to go back and shoot those dream sequences (or already did and cut them) for the Director's Cut?
So far, I can only count four known sequences (and strongly suspect four more) deleted, and they don't add up to 20-30 minutes:
1: Extended Opening (more Engineers, religious/sacrificial ritual)
2: (suspected) Shaw and Holloway's Camp (more fully sets up the characters, esp. Holloway, leads up to Shaw breaking through to the cave)
3: (suspected) Janek/Vickers Sex scene (yeah, RIGHT)
4: (suspected) Aftermath (Shaw confronts/attacks Vickers in the wake of Holloway's death, then doubles over - first indication something's wrong with her)
5: (suspected) Maternity Ward (While Weyland/Vickers have their heart-to-heart, David looks in on Shaw's "baby")
6: Extended Fifield Attack (happens as Weyland's party leaves the ship, Shaw's the one who runs Fifield over)
7: Extended Shaw/Engineer Fight
8: Extended Ending (Proto-Alien gets up and leaves the lifeboat, walking out amid the wreckage of LV-223 - the first of the planet's new race...)
Ridley could also stick the "TED2023" and "Shaw's Audition Tape" viral videos (in that order) in between 1 and 2.