@Martin
There are a few “Coming Soon” themes available on The WordPress.org theme repository. One of them is Ready to Launch . My recommendation is to use one of the “Coming Soon” plugins instead.
Why Use a Plugin?
The advantage to using a plugin is that it allows you to work on the site while the public sees the coming soon page. Anyone who is logged in will be able to see the progress on the theme. I use CSS Jockey’s Custom Coming Soon Page because it allows complete control over the coming soon landing page and includes a simple email sign up and jQuery countdown timer.
This allows my client to add content and see the progress of the site while it’s being built.
Here are a few more plugin choices