For an extensive answer you probably should ask the people at CERN IT 
directly, but so far I can tell you:

Main differences: 
- There are some other default settings for installation concerning 
connectivity and authentication
- There are different images during the installation
- There is an additional repository (non-default) available for CERN-people 
only due to copyright restrictions (some 20 packages or so)
- The distribution is (at the moment still being) tested/certified for 
functioning with all official CERN software (data analysis, detector control, 
etc...) and all CERN certified hardware.
- For CERN members there is a dedicated support for this distribution, which 
is not available for outside people.
- There are some additional software packages that are used at CERN and 
without too much use outside.

About the default RPMs I don't know.

If it is suitable outside depends what you mean by "suitable". 
If "suitable" means "it runs" then I'd say yes. (At least it was like that 
when I tried SLC5.
The question is more: Why would you need SLC6 instead of SL6?

Further information you might find at http://linuxsoft.cern.ch 

Cheers,
Bernhard

On Tuesday, July 19, 2011 10:59:26 AM Mark Marshall wrote:
> Hi,
> 
> I had a few questions about the SL6 CERN build....
> 
> What are the main differences between that build, and the vanilla SL6?
> 
> What RPMs are included by default in the CERN build, but not in SL6 proper?
> 
> Is the CERN build suitable for external usage?
> 
> Thanks for any advice,
> 
> Mark