From a question on the Japanese mailing list:

TUV is committing to a 10 year production lifecycle for 5 and 6. CentOS 
has now reflected this on their project's lifecycle page. Scientific 
Linux does not match this.

TUV:
https://access.redhat.com/support/policy/updates/errata/

CentO:
http://wiki.centos.org/FAQ/General#head-fe8a0be91ee3e7dea812e8694491e1dde5b75e6d

SL:
http://www.scientificlinux.org/distributions

Is SL going to extend support in the same way, or stick with the 
original lifecycle limits? Or... is this even something that the project 
cares to consider at this point?

I'll relay the answer, but I'm sure this question is of interest to 
others as well so I re-posted the question here in English.

-z