A few years back, an intelligent member of the Team Fortress 2 community came up with a program that completely automated the "idling" process, allowing players to collect hats, weapons and other in-game trinkets without actually firing up the game. Valve dealt with this particular method a while back, but it's only in the last couple of days it's really clamped down on the practice.
The first is to disable item drops in "text mode" — that is, running the game without graphics. By using text mode, a player could idle for items in the background while performing other tasks on their PC, thanks to the significantly reduced load on system resources.
The second is to prevent drops if you have "misplaced" items. "Misplaced" means that you've received the item, but haven't accepted it into your inventory. During regular play, misplaced items are put into your inventory during certain events — map changes, player death, etc. Basically, some interaction is required by the player to "accept" the item, something that would not occur while idling.
These changes are not mentioned in Valve's official patch notes for TF2, but that comes as no surprise. For regular players (and those who idle manually), this doesn't change anything but for serial idlers running many sessions of the game at once, it's going to cause them some consternation.