Change index.php to a login/registration form

Hi,
What I want to do is replace the initial home page with the login and registration screen so uses will not see any content until they have registered or logged in, then redirect to the original store front page.
The reason for this is that the 'store' is only to be made available to a select group and by invitation only - it isn't open to the general public, and it doesn't really sell anything (strange I know but a detailed explanation would bore you to sleep!). I'm using it as a web ordering system.. and I don't want the store content to be on public display.
Does anyone have suggestions about the best, most efficient way to achieve this customization?
I guess I want the form centered on the page with a pretty background image.

Cheers,
Dave.