I've spent the last few months saying “I'm building a sign up page.” This is basically the “New Blog” dialog in the writer/pad, except with a payment form included as well, located at /new/blog. The idea is to get more people converting to paid accounts, especially since many current customers upgraded within 0-1 days of using Write.as.
I spent a lot of time worrying about the whole thing requiring two API calls to work (one for sign up, one for payment); I really don't like the potential for the client to be in that in-between state with a bad internet connection, or some unforeseen JS error. But the registration system is already really robust, so worst-case, users will just have to upgrade the old fashioned way.
Though there were other things going on, this took a lot longer than I thought, and amazingly, I'd built things right enough the first time through to make adding this easy.