Ah, you've stumbled upon something here. If you use the Minimal CD, that would not be an issue.On another note (maybe of help), I tried installing webmin from the webmin site (http://www.webmin.com/rpm.html). The RPM has the same sha1 as the cache file referenced above. That RPM refuses to install with an error that it does not recognize the operating system. The file /etc/system-release says: "Generic release 27 (Generic)"
I will have to submit a bug report on this as it could affect other apps.