Some behavior has changed in Red Hat / CentOS 5.3. Refer to Red Hat Enterprise Linux 5.3 Release Notes.

Notably, the PATH environment variable in sudoer no longer get carried over from the normal user's shell.

Quote:

the env_reset sudoers option from newer sudo will reset the PATH environment variable. This is different from the behaviour in sudo-1.9.8. To keep the old behaviour simply add PATH variable into env_keep in sudoers file.