No, there is no definitive place to watch.  I will include a bunch of posts
I have made on new builds below.  I try to stay on top of it, but I am only
human, and have other responsibilities, like day job, family, etc.

http://sqlblog.com/blogs/aaron_bertrand/archive/2006/12/12/419.aspx
http://sqlblog.com/blogs/aaron_bertrand/archive/2007/03/06/service-pack-2a.aspx
http://sqlblog.com/blogs/aaron_bertrand/archive/2007/03/28/an-update-on-recent-s
ql-server-2005-builds.aspx
http://sqlblog.com/blogs/aaron_bertrand/archive/2007/04/04/the-drama-continues-m
ore-post-sp2-fallout.aspx
http://sqlblog.com/blogs/aaron_bertrand/archive/2007/04/11/more-on-gdrs-qfes-and
-the-aftermath-of-sp2.aspx
http://sqlblog.com/blogs/aaron_bertrand/archive/2007/05/03/some-new-or-updated-s
ql-server-2005-builds.aspx
http://sqlblog.com/blogs/aaron_bertrand/archive/2007/06/27/staying-on-top-of-sql
-server-2005-builds-and-hotfixes.aspx
http://sqlblog.com/blogs/aaron_bertrand/archive/2007/06/27/some-new-and-updated-
sql-server-2005-builds.aspx
http://sqlblog.com/blogs/aaron_bertrand/archive/2007/09/11/new-cumulative-update
-is-available.aspx
http://sqlblog.com/blogs/aaron_bertrand/archive/2007/10/16/quick-update-new-cumu
lative-update-has-been-released.aspx
http://sqlblog.com/blogs/aaron_bertrand/archive/2008/02/06/list-of-sql-server-20
05-post-sp2-builds.aspx
http://sqlblog.com/blogs/aaron_bertrand/archive/2008/02/19/i-would-rather-see-sp
3-but-i-ll-take-cu6-instead.aspx

And then these guys try to keep a list too, but everyone seems to have
little holes in their lists here and there:
http://www.aspfaq.com/sql2005/show.asp?id=20
http://www.sqlsecurity.com/FAQs/SQLServerVersionDatabase/tabid/63/Default.aspx
http://sqlserverbuilds.blogspot.com/
http://www.sqlservercentral.com/articles/Administration/2960/

Microsoft is trying to maintain a list, but the following one is only 3152+
http://support.microsoft.com/kb/937137/