It's quite simple. The easiest way is to preserve your store formatting (header, footer, nav menus) and basically put "something else" into your homepage:
1. In
/skin1/customer/home_main.tpl
FIND:
Code:
{elseif $main eq "catalog" and $current_category.category eq ""}
{include file="customer/main/welcome.tpl" f_products=$f_products}
REPLACE WITH:
Code:
{elseif $main eq "catalog" and $current_category.category eq ""}
{include file="customer/main/new_homepage.tpl"}
{* {include file="customer/main/welcome.tpl" f_products=$f_products} *}
2. Make your new homepage (call it new_homepage.tpl or whatever you want).
Now, by doing it this way, you still have 100% of your smarty vars and you can basically make anything you want (and you preserve your store's consistent look and feel)
You will be giving up welcome.tpl and all the junk it comes with (this is usually a good thing).
Use welcome.tpl as a guide -- but you can now make something that is visually unique and specific to your store.
PS -- I agree with Padraic: "splash pages" are Very Bad (and only slow your visitor down) -- unless you are selling image... but for shoppers, it's hard to justify a splash page.