I feel like the drop system was changed with the last major update. I thought I read that a maximum of 11 items per week/20 hours is alloted and any extra playing or time would not produce items.
Quote:
Current Item Drop SystemOn April 20, 2010, the item drop system was changed. Instead of rolling randomly at intervals to see if you got an item drop, now the system rolls to determine when your next item drop will occur. With this change, players are guaranteed to find items at fairly regular intervals. This was made because many players had unlucky streaks, where they didn’t find any items for weeks. That isn’t possible anymore. The drop rate was also increased, but now players have a limited playtime per week when they can receive drops. Playing beyond that amount won’t find any more items, but some of the unused time will rollover to the following week.
I mean idling still is beneficial I guess except it has diminishing returns when you do playtime vs items.
Source:
http://tf2wiki.net/wiki/Item_drop_system#Current_Item_Drop_System