Transparent Text In Cycles Not Working Before I begin
Let me state my end goal here before I continue, in case the method
I'm using isn't the best way: I'm trying to make text appear and
disappear in an animation. What I'm doing now is animating a mix
shader with a transparent node and a diffuse node. If this is the
wrong way, please correct me.

Please ignore this part. Workaround found. However, my question still stands.

I have a text object in my scene that I'm trying to switch between transparent and opaque. However, instead of getting transparent text, I'm getting a grey colored text.

I've tried the method on other objects, and it works there, but not on the text.
Any help is appreciated.

Download link for the .blend
This is because the text is in the same place as the background plane.

TL;DR: Move the text a little above the plane so they don't intersect and interfere.

Blender has no way of knowing if the text should be on top (and invisible) or if the background plane should be on top (obscuring the text). Even though either scenario comes to the same result in this particular case, such an undefined situation is liable to cause undefined behavior.

What happens is some cycles form of z-fighting (the classic z-fighting artifacts only seem to appear when in ortho view and from some angles). If you put an object behind the plane and text, the issue becomes clearer: