I love this idea. With everything on the front page, immediately usable, it would be my first choice for invoicing. If you add required user accounts and a huge site around it, I've got no reason to switch (optional user accounts obviously wouldn't bother me and I think some people would want them).

Once I've got the PDF generation done I'll work on this next.

Yup, it won't change. I just need to figure out how to tell folks that they can edit in place.

You could put the blinking caret in price field.