A thread titled invisible skin wtf !!!!!! on the Quake3World.com Forums has inspired a response from id Software CEO Todd Hollenshead on how to deal with the possibility that someone will sneak a skin into a Quake III Arena skin pack that will allow them to unfairly hide more easily (thanks Nathan Fahrenthold). Here's the deal:

This stuff has been around forever. In Quake II, people made models that were a one pixel cube and tried to get people to download them. If you're afraid of getting a "cheat" skin as part of a skin pack and aren't willing to check, then you shouldn't download skin packs.

As far as whether we should "allow" this or not, we can't prevent it without preventing all custom skins. There's no way to write a program that only uses the "non-cheating" skins and disallows all of the others. If you don't want custom skins, you don't have to get them. The game ships with 89 skins and 20+ models. And you can always use cg_forcemodel if you see someone using a skin that you don't like until you can find it and delete it.

Lastly, you can always contact the skin download sites and ask them to remove skins that appear to be cheats or at least give you the option of which skins you want to download.