Apache, when used on a VxFS filesystem, will only serve files that are smaller than 256 bytes. You can manipulate said files any way you would like to using the operating system, however, Apache refuses to serve them, instead serving up zero-byte files. An strace will reveal that the read() syscall is returning EAGAIN.
Well, it seems from the VxFS release notes that the sendfile() syscall is not supported on VxFS filesystems. The simple solution is to add 'EnableSendfile Off' to the httpd.conf, and everything will be working fine again.
UPDATE - I have received word from Symantec support that this is scheduled to be corrected in 5.0MP2, but no timelines yet.