I use this on my Mac and in Firefox all the time.
Forget password 'vaults' on your computer, a USB flash key or the web, I want an easy way to gen new passwords and recall existing ones and this does it perfectly.
It's a fairly simple looking (from my untrained eye) set of javascript that does it using two values (a site tag -- like the domain name, and a 'master password' of your choice.)
The iPhone app should let me input the two values and get the third -- the actual password.
Someday when copy/paste is actually supported this could be used to supply passwords for webapps on the iPhone, but I'll be content flipping between apps or using it when I'm on someone else's computer.