I found myself in just that situation recently. I looked around for a plugin to let me replace my index with a simple html document for a little while. I may write one yet, but for now a beautifully simple solution is doing the trick. Here’s what to do:

• Create a blank file in your theme directory and call it something like splash.php.
• Make it a page template by pasting in the following:

  <?php
  /*
  Template Name: Splash Page
  */
  ?>
  

• Write your static HTML. If you need to link to images or stylesheets, I suggest putting them in a subdirectory of your theme called something like splash. You can then reference them in your HTML using a bit of code like <img src="<?php bloginfo('template_directory'); ?>/splash/image.jpg" alt="" />. Notice the php which finds your theme directory for you so your theme remains portable.
• Create a blank page and set it to use the custom page template you’ve created. You’ll find it under the Page Attributes menu. As of WordPress 3.1 it’s on the right side of the admin and looks like this image.
• Under Settings > Reading set the front page to use a static page and select the new page you’ve created. The index of your website will now be your page template with no dynamic content and no other theme files included. Easy-peasy.

Because you never called get_header(), get_sidebar(), or get_footer() you can now have a totally static splash page while you’re tweaking a theme, editing content, and doing your WordPress business in secret behind the scenes. If you still want to pull in some content from WordPress (like the title and page content) you can even grab the loop from your existing page.php and use it in your splash page.

 <?php
// must use a variable for page id
// http://codex.wordpress.org/Function_Reference/get_page
$id = 3;
$p = get_page($id);
echo apply_filters('the_content', $p->post_content);
?>

You can get your page ID by editing the page. It will show up in the url (the ‘post=x’ portion.) It’s important to note that you have to pass a variable to the get_page() function. If you just pass an integer it will throw a fatal error. No need to go into why that is; just keep it in mind. get_page() essentially wraps get_post() so that function’s documentation is a good place to start if you want to learn what’s available to you.

This isn’t a particularly pretty solution, but it’s quick and it works well provided you know your page IDs and you’re not making a theme for distribution. I like to name my page something like ‘**Sidebar Content’ so it is easy to differentiate from regular pages.

You can read a little bit more about the plugin at its official home on this site or go straight to the WordPress plugin directory to download it. I hope you find it helpful!