Ok, someone noted that there is another mod that basically does the same thing, but also has a config panel to go with it. You can grab that from here http://wow.curse.com/downloads/wow-addons/details/togglenames.aspx. However I will leave this one up as it is extremly lightweight and works fine for people that want this particular funtionality.

NameTagSwitcher switches off Guild Names and Player Titles above characters when you enter an instance/bg/arena and back on again when you leave.

I wrote it just for myself as I didn't like the name clutter in instances and I was fed up with doing this manually, and decided to upload in case it was of use to anyone else. Since it was just designed for my own use, it is kinda specific in what it does, but is easy enough to customise with even a minimal amount of lua knowledge. I have no plans to add any options or customisation for it, but you never know. :)