The iPhone appears to have an installed base somewhere upwards of one million units according to Apple's official numbers. A majority of those units have been sold to early adopters; that is people with a certain adventurous spirit, not just those who got to the story early. Some have been hacked by hand, others through tools like AppTapp. Hard data on the number of hacked units is indirect.
I haven't been able to get in touch with "lg", the man behind AppTapp/Installer.app but I was able to talk to other developers: there have been over 70,000 Open SSH downloads and upwards of 600,000 unique visits to the SMXY repository this month. Since only a portion of Installer.app users will install SSH, that hints at a much bigger base.
What is my best guess? I'm thinking conservatively that between 10-20% of early adopters have hacked their iPhones for third party software and that a similar number are unlocking their iPhones for non-AT&T service; and, no, I'm not sure what the overlap might be. Either way, I expect the software-hacks to plunge as we enter the holiday season where most purchases will be from less adventurous customers and the unlock percentages to rise as the iPhone hits Europe.
Thanks to Shaun Erickson and Nate True.
Update: The fabulous Drudge has uploaded a complete breakdown for his most popular hosted packages. In addition, he reports nearly 800,000 unique visits between the Conceited and SMXY repositories. Even taking EDGE into account, that's a lot of visits. "Kroo" lent his able assistance to the stats gathering effort.