The Forsaken have already carpet bombed Southshore with plague, so their annual ritual of tossing innocuous stinkbombs into town probably isn't going to have much of an impact anymore. We aren't sure how Blizzard is planning to update Hallow's End for Cataclysm yet, but the patch 4.1 data has some hints as to what we can expect this year -- at least the fun and games portion, if not the lore.
Mixed in with the latest additions to Wowhead's database are new versions of the Hallow's End masks. Rather than replacing your helm slot, it appears that the masks now go over your helm slot -- meaning your human female can wear her tauren male mask while raiding Blackwing Descent without losing the stats on her epic helm. As an added bonus, it appears the masks that aren't player races also supply a stat buff of their own. The Naga Masks, for example, are consumed on use and provide an intellect buff.
Hallow's End rewards are also likely moving over to a currency system, with the addition of Candy Coins. The aforementioned masks will likely be some of the items you can buy with these coins, but we haven't seen much beyond that.
Finally, and this one is a long shot, two very interesting spells have appeared in the data files: Greymane's Resolve and Decree of the Dark Lady, for the Alliance and the Horde, respectively. These spells, which scale with level, are buffs that replace elixirs or flasks for their duration. Either we have a worgen/Forsaken event in our future independent of Hallow's End (which we may speculate on at a later date), or the new story of Hallow's End will focus on a clash between the worgen and the Forsaken.
World of Warcraft: Cataclysm has destroyed Azeroth as we know it; nothing is the same! In WoW Insider's Guide to Cataclysm, you can find out everything you need to know about WoW's third expansion, from leveling up a new goblin or worgen to breaking news and strategies on endgame play.