The iPhone installable application does require you to go through installation process at least once to get their installer on your phone. Fortunately, they are offering what they describe as a "single command" installer to jump start the process:
New! We now have a single command Mac (PPC & Intel) installer for Installer.app, get it here. This will automate the entire jailbreak and Installer.app installation process for your 1.0 or 1.0.1 iPhone, including automatically downloading the right firmware files. Be sure to read the short README file for installation instructions.
If possible, it is recommended you do a full restore of your iPhone using iTunes before installing this, but this is not required -- it will simply make for a cleaner install.
It's in Beta and comes with no warranty, so proceed with caution.