Jim... I just wrote a post about this very issue: http://squeejee.com/blog/2009/10/29/charging-from-day-1-geni...

Are you writing all your own payment code? If so, why? I would recommend using something like Spreedly or Chargify (we use spreedly with TweetSaver). Getting payment integrated using Spreedly went very quickly for us. You should be able to get it up and running within a day or two.

I always vote in favor of launch early and iterate... but with tools like spreedly it's only an extra couple days at most to get payment plans in there.