I cannot stress the crtical importance of making a full backup of your files and database before attempting to upgrade WordPress. Mistakes do happen, and things do break. Both are shortly followed by you banging your head and cursing. Save yourself the headache…Backup first, then upgrade! 🙂
I should also mention that these files are being downloaded directly from the WordPress Trac system. I did not package the zip.
WordPress 2.5.1 was released.