Binary package hint: ubiquity
I entered 12 into the hostname field and it was accepted. A simple check that the provided hostname is at least not all numbers would help reduce incidence of invalid hostnames (which results in lots of invalid bugs against postfix when it declines to configure itself with an invalid hostname).
ProblemType: Bug
DistroRelease: Ubuntu 10.10
Package: ubiquity (not installed)
ProcVersionSignature: Ubuntu 2.6.35-20.29-generic 2.6.35.4
Uname: Linux 2.6.35-20-generic i686
Architecture: i386
Date: Thu Sep 9 12:36:30 2010
InstallationMedia: Kubuntu 10.10 "Maverick Meerkat" - Alpha i386 (20100909)
ProcEnviron:
LANGUAGE=
LANG=en_US.UTF-8
SHELL=/bin/bash
SourcePackage: ubiquity
For what it is worth in Ubiquity version 2.9.15 using a number doesn't work unless you leave the hostname a number and then enter a password in the password field (see bug 928447). So people may be discouraged by the hostname already exists on the network message and change their hostname to NaN.