Rather than simply adding some strap-ons to the Falcon I, they're working on the Falcon V that will be powered by 5 of those engines. They're supposed to be recoverable and reusable, too. The Falcon V had the potential to launch up to 5 people into space. By way of comparison, the Russians and the Chinese can only launch up to 3 people in their manned spacecraft, and it wasn't until the Shuttle that the US could launch more than 3 at a time (the Shuttle can launch up to 7 people). From what I've read, SpaceX intends to actively go after the $50 million prize offered by Bigelow for a reusable manned orbital spacecraft. Rutan seems interested in going after that one, too. Bigelow is sponsoring the prize because he wants a non-governmental way to get people to and from the private space station that he's working on.

Interesting times.