Yesterday when I downloaded it on my iPad it appeared as Facebook was the only way to login? Maybe I was incorrect and I simply missed something. If so, I apologize for my statement.
With that being said... I do love the app and think you're on to something big.
For field testing my iPad apps (for instance) I will just hand them an iPad and not give them any instructions or hints on how to use it. And then I will watch intently what they do and what they don't do. Two minutes of unsullied first impression is pure gold. The only 'downside' is that occasionally you learn that you have to redesign the interface...