The WordPress 2.9 release announcement post lists a lot of improvements and new features for theme development, however it doesn’t tell much how you can go and apply them. Here I will list some tutorials and resources that I can find about the new features.

The Canonical Post Thumbnail Feature

A complete tutorial at Kremalicious. Teaches you how to activate it for your theme, how to customize the thumbnail dimension, and so forth.

The Canonical Post Thumbnail Feature, in Your Feed

Here’s how you show the post thumbnail in your site’s RSS feed. It uses the same function (get_the_post_thumbnail()), so you’re still able to set the dimension and so on.

Register Theme Support for WordPress Features

the new add_theme_support allows your theme to declare that it supports certain features (one of it is the Canonical Post Thumbnail above). Learn how to use it here.

Sidebar Widget Area Description

Justin Tadlock has it covered in this post.

Remove and Modify the ‘[...]‘ Text at the End of an Excerpt

The Codex is the way to go. Also shows you how to do it for WordPress version 2.8 and below.

Extend the User Contact Info

Add new fields for your user’s contact information (say, to allow them to input their Twitter account). I find it a little bit strange that a feature tied into Dashboard activity (user information input) needs to be activated by codes inside the function.php file of a theme, but there you go.

Order Posts by Comment Count

query_posts() now supports ordering by comment count, an easy way to display the most popular posts.

Customize the Separator between Tags in a Tag Cloud

wp_tag_clouds() now allows you to specify your own tag separator.

That’s all I can find for now. Anything I missed, please let me know at the comments. Thanks!