Frankie at Bungie.net writes that the planned playlist update is being delayed until next week (probably) while they continue to investigate the problems some users are having in downloading Halo 2 maps.

For what it's worth, I've been able to get the maps now, both on my old Xbox console and on my 360. What I still can't do is get all the old map packs on the 360; when attempting that, I get the continual "installation has failed" error with the unhelpful link to Xbox Support that others are getting while trying to download the newer maps.