Is the Tango Desktop Project dead? It looks totally abandoned to me...
Last time the site was updated was 2009, but that doesn't mean it's dead. Infrequent updates don't men a project is dead just that it doesn't have a recent update.
Tango Desktop is quite far reaching, and has in fact influenced many things on today's DEs.
Remember Tango Desktop is more of an initiative then it is a piece of software. So you kind of asking if html 4 is dead. It might not have been updated in a long time, but it's far from dead.
Many, many (matter of fact most) modern projects follow the goals of the Tango Desktop Project. As do "most" DEs and modern UI guidelines.
The project is still important, still relevant, but just doesn't need a bunch of updates to keep it so.
Looks alive to me. Go into Bugzilla and search for tickets/issues related to the Tango project and you'll see that people are still working issues. That and code check-ins are usually the best 2 indicators if a project is alive or dead.
The mailing lists and IRC are probably the other best 2 indicators. I looked through the mailing lists and they're ghost towns but this isn't something that requires a lot of chatter either, so as others have said in comments and the other answer, lack of updates and low frequency don't mean a lot on a project that's providing foundation layers for GNOME.