I have seen quite a few tutorials that offer an easy snippet of code to hide the WP version number in the <head> of a theme, however they rarely include a way to disable it from appearing in feeds generated by WordPress.
Here's a quick tutorial on how to solve that issue:
http://skattertech.com/2011/01/how-to-properly-hide-the-wordpress-version-number/
Also, it offers a .htaccess tip to disable public access to the readme.html file which also contains the WordPress version number.