This was just added to the list of issues with the Unreal Tournament demo over at Epic's Unreal Technology site (thanks Frans):

Due to an oversight, the patch does not add the OpenGL or S3 Metal rendering drivers. If you have patched the 3dfx demo and really want to try these drivers, you will need to download the full 55MB demo. You are still best off using Glide for your 3dfx card, or Direct3D for all other cards except S3s.

A list of mirrors is available below if you need to download the full demo.