Oliver McFadden, the developer previously behind the Revenge program for ATI reverse-engineering, previously brought the ioquake3 engine to the Nokia N900 with OpenGL ES 2.0 on the PowerVR SGX graphics core. A few months back he then started on a new GLSL back-end for the idTech4-based Doom 3 open-source engine and OpenGL ES 2.0 and EGL support for this once-popular game.
Oliver McFadden wrote this morning today on the Phoronix IRC channel (#phoronix on freenode.net), "michaellarabel: I've released the Doom 3 ES2.0 code and GLSL shaders (GPLv3.) There are still a few bugs and unimplemented features so it's more for developers at this point..."
The code for McFadden's modified ioDoom3 engine can be found in his idTech4 GitHub repository. This provides initial OpenGL ES 2.0 rendering support and the GL Shading Language (GLSL) renderer back-end.
Oliver additionally said on Phoronix IRC, "the default configuration is to match the Doom 3 ARB2 renderer pixel-for-pixel (i.e. Blinn-Phong) but I've also implemented the Phong shading model...at least as much as possible within the constraints of the existing media; Doom 3's specular maps are really gloss maps so the specularity power is still a fixed value...you could probably get some nice improvements by adjusting the factor based on the surface type (similar to how you have different sound effects for bullet hits)."
In terms of upstream improvements for the ioDoom3 engine, there sadly doesn't appear to be anything particularly exciting going on lately. If you're wanting to find a visually-impressive game to work on right now, it's worth looking at Unvanquished.