Last month, the root-requiring Superuser app got updated to 3.0.4 and then was having all sorts of problems. A temp fix was posted that set it to allow all requests and thus prevented it from crashing. At the time, it was the only way to run root applications if you were stuck with version 3.0.4.
So many allowed all requests and vowed to never upgrade Superuser again, as the previous version had worked flawlessly. Subsequent versions have been released that fix all those problems, but those who vowed to never touch it again are still universally allowing all root access requests.
I don't think many people thought about it, and I know I certainly wouldn't have thought two twice about an issue that was resolved weeks back, but then a game I reviewed last week had an update, and all of the sudden it was requesting root access for no apparent reason.
So, if you used the temporary fix and decided not to update your Superuser, you might want to grab the new version in the Market and make sure allow-all is turned off. I've been running the latest version (3.0.6 as of this writing) since it came out and have not had any issues.
If you don't update Superuser or undo the temp fix, any malicious application you download could potentially do bad things to your phone with the root access that's granted to it. From stealing information to dialing 900 numbers, they might even order bulk chocolate from your phone to kill your dog.
Note: While I don't suspect that Zombie Farm is malicious, I currently have emails into the developers and would advise against farming zombies until they get back to me.