In an order to clamp down on Steam scammers, Valve is introducing "trade holds," which will insert a three-day waiting period between the conclusion of a trade deal and its actual execution. The idea is to give Steam users an opportunity to discover that their accounts have been compromised, and to do something about it, before their inventory is traded away.
What happens, simply put, is that some jerk gets hold of your Steam account, and then trades all your stuff to a second, fake account that's also under his control. Because trades happen immediately, your items are gone, baby, gone—and because Steam support is so notoriously slow, the odds of getting them back are not necessarily good.
Thus, trade holds. "If a user trading away items hasn't had their account protected by a Mobile Authenticator for the past 7 days, items delivery will be delayed by Steam for up to 3 days. This provides the user time to cancel the trade and any other that are pending," the Steam Support site explains. "Canceling trades that are pending or in a trade hold will begin a trading cooldown on your account to prevent any further unauthorized attempts to trade away items."
Using a Mobile Authenticator will eliminate the need for a trade hold, so obviously this is also a way of encouraging people to make use of them. Either way, this will be a boon all around: Your Steam stuff is better protected, and Valve will (hopefully) see a reduction in support requests for lost accounts or items. Given Valve's recent (and repeated) admission that its customer support is a gong show, reducing the demand for its services has to be a priority.
The Counter-Strike Blog says Steam trade holds will go into effect on December 9. To encourage people to get into the swing of things, Valve is also offering discounts of five to 33 percent on purchases from the Steam Community Market for anyone with a Mobile Authenticator until 10 am PST on December 16.