Dear Miroslav,
An option would be to include all packages from the newly created Spacewalk $ver-client repo, e.g.:
Not in CentOS5 since recent this version require python-dmidecode which is not in RHEL and therefore probably in CentOS.
Ah, I forgot about that.
Partialy true. Recent upstream version will not hit accidentaly rhn.redhat.com That version included in RHEL will do that by default. But these change: https://fedorahosted.org/spacewalk/changeset/cd925d940357d145c7106c1c3ed6ed1... is only one which are required to prevent from accidental check of rhn.redhat.com
So my recomendation is: take this patch and apply it on current rhel5 package and include all of them in CentOS.
I would personally do not include the rhns client packages to use them with Spacewalk.
Spacewalk development is much faster which may lead into missing features as they are not yet implemented in the version that is meant to be used with rhn.
The only option I see is to keep the rhns and spacewalk-client packages in sync, which may be the case in the near future (e.g. on RHEL 6)
Best Regards Marcus