A Steam Community announcement details changes Valve is making to the ability to trade gifts on Steam to curtail the abuse of the system by third-party key resellers. Here's what's different:

All new games purchased as a gift and placed in the purchaser's inventory will be untradable for 30 days. The gift may still be gifted at any time. The only change is to trading.

We've made this change to make trading gifts a better experience for those receiving the gifts. We're hoping this lowers the number of people who trade for a game only to have the game revoked later due to issues with the purchaser's payment method.