paying attention to online game stats for StarCraft 2. Overall, the Protoss faction seems to have an unusually high win percentage against Terrans in certain leagues of play. Furthermore, while Zerg statistics don't indicate any huge disadvantages, Blizzard notes in the Battle.Net blog that "the feedback from the community as well as our own play experience tells us that improvements are necessary to make Zerg matchups feel and play better."
The studio states that it is currently "working on solutions" for the Protoss vs. Terran issues. In the meantime, the next patch will make adjustments that should benefit Zerg players. See the planned changes after the break.