you need to attend a special class on what themes in wp nightly builds is all about.
I cannot start that class now, but I will give you some things to ponder over.
In WordPress 1.2, the same index.php was used when the default page, category page, monthly archive page, or a single post is viewed. And also the themes that you can create for WP 1.2 are merely a change of the stylesheet.
so switching a different theme in WP 1.2 meant, only loading a different stylesheet, but still using the same index.php
you can see it in action at my website http://wpthemes.info ' s homepage, look at the bottom of the sidebar.
In order to differentiate whether the index.php is being queried for a single post, or category page, or monthly archive page,
Kubrick from BinaryBonsai.com came up with an idea.
For example, to see if it is a category page, Kubrick looked for empty($_GET['cat']) and empty($_GET['category_name']) .
this is for WP 1.2 .
but when WP 1.5 was developed, the developers took Kubrick's idea and made it much simpler and provided another method to determine the same thing.
they came up with "is_category()" , "is_month()" etc to make it easy to code and easy to understand.
this method is only available in the nightly 1.5 builds.
So if a theme uses a is_category() method and if you try to use it for wp 1.2, which has no idea about this new method, its going to break obviously.
Thats why WPThemes.info has put special category for the themes.
all these downloads clearly belong to 2 categories.
"Downloadable Theme and WP 1.2" for 1.2 themes (i should say 1.2 styles)
and "Downloadable Theme and WP 1.5 for 1.5 themes.
and a 1.2 theme may not work on 1.5 and vice versa.
I hope this clears some of your views. I can still go on this topic, but I will do it sometime later.