I've been pushing Balsamiq Mockups for a number of years, it was on my toy list for 2009.
Once you have a mock up of your wireframe and the customer is happy with the look, the next stage is to convert it to HTML, previously I have prefered to pass this to a designer to make it look pretty. But the problem you'll have with making something look nice is that the customer will think it is DONE and you just don't want that to happen.
I came across a Balsamiq Mockup to HTML/CSS converter this morning from NedeCo it just does the job and generates a simple and easy to follow HTML and CSS.
This is a perfect starting point for your application and you'll be able to get your business and architecture up and working before the customer gets any expectitions of a DONE project.
BalsamiqConverter.zip (511.03 kb)