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