New Steam Guard feature to help protect your items
During our server maintenance on August 28th, 2013 we implemented a new security feature to protect users from having their items stolen. As a result you may receive a message that says you have logged into a new device and are unable to trade for 15 days. If this is the case you will need to wait the 15 day period before you can trade from that device/browser, because you are on a device/browser that has not had Steam Guard enabled on your account for 15 days. Steam Support is unable to remove this restriction from your account.
You can continue to trade from any devices or browsers, that were authorized for at least 15 days.
To avoid this restriction from triggering you should not delete Steam cookies from your browser, deactivate Steam Guard on your account or reinstall the Steam client. If you have Steam Guard enabled currently and have had it enabled for 15 days on your account then you will be unaffected by this change.
Our goal is to prevent trading on your account before you are able to react if your account is hijacked.
Hopefully this will stop all the account hacking.