Another company has started selling tickets for a sub-orbital space flight. After reading that article I started wondering why they don't just enter an orbit around the earth. That would be a more exciting trip and it would last longer. If you're paying 150,000 dollars, then you don't want the trip to be over in just a few minutes or hours. So I did some research into the difference between orbital and sub-orbital flight. As usual, Wikipedia had a very good article on the subject: Difference between sub-orbital and orbital spaceflights. Turns out that the lateral velocity of the ship is the big difference. Here's an excerpt:

The difference between the lowest speeds required for orbital and sub-orbital space flights is substantial: a spacecraft must reach about 18,000 mph to attain orbit. This compares to the relatively modest 2,500-3,000 mph typically attained for sub-orbital crafts.

The important difference in energy requirements between a sub-orbital spaceflight such as that required for the X Prize and for an orbital spaceflight is that no lateral or angular velocity is required for the sub-orbital flight. The energy required to get to 100 km or even 350 km altitude is dwarfed by the energy required for the necessary lateral velocity of orbital space flight.

In terms of energy: accelerating a spacecraft to orbital speed requires about 31 times as much net energy as just lifting it to a height of 100 km (together 32 times).