How is this done?

a 3d perspective projected point doesn't make much sense. sure you don't mean a 4d vertex in clipspace? (ie after it has been multiplied with modelview and projection...)

the process of mapping clipspace coordinates to sceenspace coordinates is usually called viewportmapping, and opengl defines it as something like this

sx = vx + ((x / w) + 1) * (vw / 2)
sy = vy + ((y / w) + 1) * (vh / 2)
sz = ((zfar - znear) * z + (znear + zfar) / 2

where vx, vy, vw and wh are viewport x, y, width and height.
the output (sx, sy and sz) are device-space coordinates.

hmm... maybe a I need to be a little clearer

I have a camera positioned at (0,0,-10) with positive Z-values going into the screen. I want to place a texture centered around and located at (0,0,0) that will have retain its dimensions of 128X128 on the screen.

I asked how I can convert the 3D point in space into 2D screen space so that I could work from there in figuring out how to make the 128X128 texture placed at (0,0,0) retain its screen size.

I think kusma may have answered this whole question, but I have not had a chance to try the solution out. Kusma, is the information you posted going to solve this? Can anyone else help?