As a professional web developer I wouldn't want to create an entire site using my iPad. There are a few very good apps with syntax highlighting and ftp support which is great for making changes to existing sites.
As someone has already stated assets such as images could be a problem. While its doable its would be very time consuming to create even a simple image of a select size containing text.
Another option is a hosting company who provide a web based site builder. These are quite limited but would be enough to build a basic site using Safari (as long as its not Flash based).