Nice work for a weekend product. How does your system handle spam? e.g. if I schedule a tweet every 1 second to 20 accounts that may get your oAuth or ip blacklisted from Twitter.
I could only see what the Pro cost is very small at the bottom of the screen. You should make your pricing structure more obvious, and I would like to select which version im signing up for before I sign up for it.
I'm going to play around with where else to put the price as you're right, it's a bit hidden at the moment.
Currently, the only way to pay is to log in with a Twitter account first and then upgrade from there. I'm thinking about an option to "sign up for premium" where after signing in with Twitter the user's taken to a payment page. Do you think that'd be a good idea?
Then like you say, take me down a altered registration path if I chose pro.
I am currently working on my own Tweet Suite, shall have to let you know when it's ready so you can repay the favour ;)