Geometry only has the visibility on or off attribute, if you want gradual visibility change you're going to have to take to a shader level and duplicate the shader and animate the transparency, I don't know how else you would do that in Maya. If you can do it in post then you have more control.

- Render out each letter in a its own render layer and animate layer visibility in a compositing package.

- Render out the logo on one layer and use animated masks in a comp package.

My $0.02.


Ahh dave beat me to it lol