http://www.teamfortress.com/post.php?id=3727

Quoted from teamfortress.com:

"Today we're shipping some significant changes to the item drop system.

Previously, we rolled randomly at intervals to see if you got an item drop. Now we roll to determine when your next item drop will occur. So you're guaranteed to find items at fairly regular intervals. The bulk of drop complaints we've received in the past have been from players having unlucky streaks, where they didn't find any items for weeks. That won't be possible now.
We've significantly increased the rate at which item drops occur, compared to the previous average interval.
There's now a maximum amount of playtime per week in which you get item drops. Playing beyond that amount won't find any more items. The amount varies, but if you play an hour or so a day you're good. Some of your unused time will rollover to the following week, so if you're an intermittent player you'll also be fine.
What are the net effects of these changes? Here's the quick summary:

If you're an idler, you're going to find fewer items than you were prior to this change. Sorry. On the bright side, there's less reason to idle, because it won't earn more than players who are playing regularly.
Everyone else will find items more frequently, and with greater regularity.
We're still thinking about how to allow you to influence your drops. When we see discussions on the forums, people often ask why we don't tie it to some in-game performance (like your scoreboard position, or number of kills, etc). Many players understand that if we did this, idle servers would simply change into servers running plugins that generate those in-game events frequently (as we saw in the Soldier & Demoman WAR!).

There's another side of it that we care about just as much, which is the message it would send to all the players who don't want to idle. Any in-game performance metric we chose would result in there being specific maps, classes, tactics, and so on that resulted in more drops. We really don't want that to happen. When it's simply playtime, you're free to play the game however you like. If you love Payload, you can play Payload maps without worrying about whether you're earning less drops than the guys playing Arena maps. Community mapmakers don't have to worry about whether their maps earn item drops fast enough. Similarly, server operators can configure their servers however they like, without worrying about reducing the rate at which their players are earning drops. In short, while it's far from perfect, not tying to in-game performance is a lot less poisonous to your minute-to-minute game experience."

More info:

"Your time is valuable, and the fact that you choose to spend it on Team Fortress 2 is awesome. When we introduced the Item Drop System back in 2009, one of our goals was to reward you for spending your time with us.

We recently made a change that helps ensure players are playing TF2, not just leaving their computers on in order to get item drops. As of last week, players can no longer receive item drops unless they are actively playing the game.

As with any update, this change is meant to keep the game as fair, fun, and rewarding to as many players as possible. We will continue to examine how the Item Drop System is used, and make any adjustments as needed to benefit the players.

Questions and Answers:

What is the definition of “actively playing” Team Fortress 2?

From an item drop perspective, a player is considered to be actively playing if they meet all of the following criteria:

Is connected to a VAC secured server.
Is responding to in-game drop notifications while not utilizing external programs to do so.
Is running only one instance of the game.
Is not in textmode.
A player who meets all of those criteria is earning time towards random item drops (subject to the weekly cap).
Will I be VAC banned for not actively playing the game, also known as ‘idling’?

No, you will not be banned for just idling."