From the official WordPress Blog:

As you know over the past couple of months we have been working on the new features for WordPress 2.9. We have also been working on trying to make WordPress as secure as possible and during this process we have identified a number of security hardening changes that we thought were worth back-porting to the 2.8 branch so as to get these improvements out there and make all your sites as secure as possible.

The headline changes in this release are:

• A fix for the Trackback Denial-of-Service attack that is currently being seen.
• Removal of areas within the code where php code in variables was evaluated.
• Switched the file upload functionality to be whitelisted for all users including Admins.
• Retiring of the two importers of Tag data from old plugins.

We recommend that you upgrade your site to this new version of WordPress to ensure that you have the best available security protection.

To upgrade, you can use the upgrade feature built into the WordPress admin panel, or contact us if you require further assistance.

This is very useful for those of you Windows users who want to run a WordPress site locally on your machine; whether for development purposes, or otherwise.