A small update is coming to StarCraft II today, May 10, 2012, which contains three balance changes:

Protoss

Observer build time decreased from 40 to 30 seconds

Zerg

Overlord movement speed increased from 0.4687 to 0.586
Queen anti-ground weapon attack range increased from 3 to 5

These changes have been tested for the past week, an will soon be applied to all games and not just one map. There won't be an actual "patch" for these changes; there will only be a short maintenance in each region prior to the mini-update, and you won't have to patch to update your game.

Source:
Battle.net - Balance Update