Valve is introducing a three day hold on all trades between people that do not have their Steam accounts secured with the Steam Guard Mobile Authenticator. Valve recently tossed up a new Support page that details how you can prevent the 72-hour hold on all trades. It's actually quite simple.
What is a trade hold?
A trade hold is a period of time where the items traded are held by Steam before they are delivered.

Why are trade holds necessary?
Trade holds protect your items. Because Steam accounts are valuable, especially if they have items worth stealing. If you haven't protected your account with a physical device (using the Steam Guard Mobile Authenticator), a trade hold will give you time to discover your account has been compromised and to prevent items from leaving your account.

How does it work?
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.

Cancelling 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.

How can I remove the need for a trade hold?
Using a Mobile Authenticator ensures that you and only you can trade away your items quickly and securely, so trade holds are no longer necessary. Increase your account security by getting the Steam Guard Mobile Authenticator for iOS and Android devices.

Valve is going for the big one-two punch here with trying to eliminate having to deal with stolen accounts and having to deal with stolen items in a shady trade deal. Unless you're using a Windows Phone (or don't have a smart phone at all), there really is no excuse not to get this set up right away.

This hold policy will go into effect on December 9. It would be a smart idea to get set up with the Authenticator by December 2 if you hope to keep trading without issue on the 9th (see "How does it work?" in the above FAQ).

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