Web designer and css coder for app

We're building a web app in GWT that provides invoicing, contact management
and related functionality.

We've been working with wireframes so far, but need a designer to give the app it's final style, and express this in css.

We're looking for a cool, modern, professional look that'll appeal to the facebook and iphone generation. No unnecessary bling!

The first stage will be mocking up some visuals to pin-down the look.

Second stage will be converting this to css and working closely with our developer to apply it to the wireframe. We must stress that we expect this to be a collaborative process, you can't just throw html/css over the wall to us - which is why we're after an hourly rate rather than a fixed price - we'll need you around to work with us on this one.