I take part in a weekly D&D Encounters program and the question often comes up. What are the "Twitter Buffs", and this is quickly followed by "how do they work"?
If you follow http://twitter.com/#!/Wizards_DnD (possibly more specifically, #dndenc) on twitter, it sometimes tweets random buffs. You can then apply them to the combat rounds you are playing in. For example, last one so far has been:
Shreds of spiderwebs catch on a breeze and flutter around your heads, clinging to your faces. PCs are -1 to hit lizardfolk.