To quote the announcement for this WordPress upgrade:

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 would recommend that all sites are upgraded to this new version of WordPress to ensure that you have the best available protection.

So,who wouldnt want to upgrade if this is, mainly a security upgrade? I know who. Those who arent all that flash with WordPress. I mean, things can go wrong, what if I mess things up? I dont even know what I am doing! Simple answer to these people is this. Backup your wordpress, then follow the tutorial that I have posted below. It will do you the world of good, and prepare you for what to expect when the big upgrade happens when 2.9 is released!