The aspect ratio of the orthogonal matrix (e.g. width/height) needs to match the aspect ratio of the viewport...otherwise you get stretching.. not sure if that is the issue....
If you are creating an orthogonal matrix that ranges from -0.5 to 0.5 for left and right planes, you need to multiply the -0.5 and 0.5 by 0.5859375 (600/1024) for the bottom and top planes to account for the aspect ratio. Maybe you are, but you didn't mention that.
If this part isn't right, it could mess up how your adding the half texel offset to y as well. Just a hunch, I didn't try doing the math.
- @Matthew: My ortho projection is simply -0.5 to 0.5, and the view has an offset of 0.5 so that the 'model' deals with 0..1 as the range. So, no, the aspect ratio really doesn't come into it. Thus, it would just stretch anything I render to fit the screen, regardless of the resolution. Why do you ask?