Cliff Bezenski is one of the map developers for UT2003/UT2004 and he added an easter egg to one of the maps for both these games.
Here's how to find it:
1. Load the map DM-Asbestos.
2. Locate the room with the lightning gun.
3. Run up the ramp that leads to the four adrenaline pills.
4. Stand next to the wall where the pills are and look straight up.
5. Now you need to deal a certain amount of damage to the roof. I recommend using the rocket launcher combo. (Holding down the alternate fire button and clicking the primary fire button once while loading rockets into the weapon. That makes them move in a spiral instead of a spreading pattern.) If you get the message "Ownage" in the message log, you will know that you've succeeded.
6. Now you have to act fast as this is only visible for a few seconds. Run to the place where the shock rifle is. (You can buy time by jumping over the fence instead of running down the ramp.) (If you really want to buy time you can use the cheat code "playersonly" (without the quotes) to freeze the time.)
7. If you now look up you will see that one of the walls (the one with the pipes) is now cut of and the empty space is replaced by a picture of Cliff Bezenski along with a caption. (Both the picture and the caption are different from a similar egg in UT-99). After a few seconds the missing part of the wall will slide back and hide the picture.
This is actually a double easter egg as it's both a hidden image and a reference to an earlier part of the series.