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.