I just found out which package takes care of the /etc/release file: it's SUNWsolnm*. So, this file actually will get updated when a new release comes by.

$ cat /etc/release 
                      Solaris 10 10/08 s10s_u6wos_07b SPARC
           Copyright 2008 Sun Microsystems, Inc.  All Rights Reserved.
                        Use is subject to license terms.
                            Assembled 27 October 2008
* SunSolve account needed