I'm not sure this issue was caused by WordPress, as it affects all files hosted by my webserver, but since I upgraded to WordPress 2.2, every single PNG file I have on my webserver is unusable.
When viewed, the image says "The image “(Image URL)” cannot be displayed, because it contains errors." If I download the image (Via FTP), the downloaded file is equally useless.
This happens to all files, anywhere on my server, even when there is no .htaccess file affecting that file.
I'm not really keen on how HTTP servers work, but since this issue coincides exactly with my upgrade to WordPress 2.2, I wanted to make sure that this issue couldn't have been caused by upgrading, before I asked my host for help.