Notice that I did not say "When did Death Ward stop the vorpal effect in this game?". I said "Since when does Death Ward stop the vorpal effect?" The answer, of course is
NEVER.
The
death ward spell only protects you from magical death effects, death spells, energy drain and negative energy effects. Ask the Sage.
http://www.wizards.com/default.asp?x=dnd/4ask/20080409a
http://www.wizards.com/default.asp?x=dnd/ask/20070725a
I seem to remember this being discussed to some length in the past, with certain people beholdent to the concept that the vorpal ability is a "death atteck" or "death effect", which it clearly is NOT. Not like the devs care...they can't even figure out how to implement beholders properly after 2+ years. I guess this is off topic though.
As for the undead, I meant in the new raid(s) explorer area. I believe they cast Death Ward on themselves and you can't disrupt them.
Blanket immunities have always been a cop-out for the developers of this game to artificially inflate the difficulty level. They take away PC's abilities, which is one of the cardinal no-no's when it comes to DMing a campaign. "Boss" fights are one dimentional hack fests. Giving them spells or something that prevent effects is perfectly fine for me, as long as we have the ability to do something about it (i.e. mantle of invunerability, globe of spell immunity, spell turning, ect). That's a much more appealing encounter in my opinion.