"Real fast, before I start, we haven't had really any hard overnerfs other than Pantehon in recent history. We also have gotten a lot better about this.
Teemo's new Move Quick essentially removed the ability to do anything against him when split pushing. I don't mind if he can split push, but it should be possible to kill him if he stays too long, or you send a strong assassin or fighter. What we're doing will probably involved the new Move Quick numbers, and probably time on Mushrooms."
-Morello, Lead Champion Designer, via the official forums.
UPDATE: Looks like I was thinking about the mushroom duration in a different light. Morello posted a preview of what the current changes are slated to be.
"Hey guys,
Here are the currently slated Teemo changes;
- Noxious Trap
- Added new visible reload timer, showing time until next trap is ready.
- Mushroom reload time increased to 35/31/27 from 30/26/22
- Damage reduced to 200/400/600 from 250/475/700
- Fixed a bug that calculated the AP coefficient lower than intended
- Move Quick
- Cooldown increased to 22 seconds from 13
These changes are pretty much targeted at high elo play, where coordinated split-push Teemo is too good at split-pushing without risk. Other play levels will likely find Teemo pretty similar to his live status now.
The AP coefficient bug fix is actually about a .2 AP ratio buff on him. This evens out the lower base damage on mushrooms.
Increase on Mushroom Reload time is so that his mushrooms are a little less carpet-bomb effect, which is cornering off entire quadrants of the map as inaccessible by the enemy team while split pushing. Move Quick is mainly to stop chaining it multiple times in single chases. We thought it was better to limit the cooldown and total mobility rather than the in-combat use, which is one of the ways Teemo can survive real fights.
We don't forsee more being done to Teemo than this, as it should retain his combat ability while let him remain effective at map control and split-pushing (just not quite -as- effective). "

-Morello, Lead Champion Designer, via the official forums.