Didn't know Sendible... looks like they've taken the concept to the max. Interesting! Thanks for the link
Don't let the existence of sedible discourage you. They are too wide in focus, while you are smaller in focus and you have a better chance of getting more tweeter users than sendible.
I agree. They have less than 9K registered users so far. I bet theres a whole lot of ppl who want to schedule thier tweats. Your strong point is that you do it only for twitter.