From December 12th, trading through the Steam inventory will be restricted to accounts that have had Steam Guard, Valve's account protection system, activated for 15 days.
Steam's trading service lets users exchange items from different games, as well allowing for the swapping and gifting of the games themselves. Mostly, of course, it's used to facilitate Team Fortress 2's strange hat-based economy.
So what's brought about the change of policy? For starters there was the recent allegation that Russian mobsters were using TF2 to launder money by purchasing keys in bulk, trading them for earbuds, then selling them at a slightly reduced price. Perhaps more tellingly, the change is being made just before Christmas, when Valve traditionally likes to perform weird experiments with sale achievements and tradable items. As this Reddit thread points out, last year crafty users were able to exploit the coal promotion to get more favourable trades.
Steam Guard is a free service that forces an additional email confirmation every time you log in from a new PC. Tying an extra layer of protection to virtual economies is becoming an increasingly common practice - Blizzard already require Battle.net Authenticator for Diablo III players looking to use the real money auction house. If you're a regular Steam trader who's yet to enable Steam Guard, you've until tomorrow to make the switch and ensure uninterrupted service.
In other trading news, TF2 recently doubled the size of its maximum backpack size to 2,000, provided players are prepared to spend the £47.43 it would take to purchase enough Backpack Expanders to reach the limit. That might seem overkill right now, but in the future, when all goods and services are purchased through a Bill's Hat bartering system, you'll be glad of the extension.