Hey David, good question. In fact I had posted asking for comments of single opt-in vs. double opt-in.
We currently use 1ShoppingCart, which has not only the ability to send out broadcast emails, create opt-in forms, etc. but also adds a shopping cart and e-book management. Sounds like Constant Contact might be a good route to go for you. It saves you from having to create forms and databases, and instead does it for you. I have heard nothing but great comments on it.
So in a nutshell, we didn't set it up ourselves. And I'm not familiar with Flash-based forms. I believe the cost you will save subscribing to something will save you a lot of headaches in the long run.
But I'm a single voice here, and maybe some web developers out there have more of an isight of "doing it yourself".

