Matrices aren't communitive. You MUST multiply the first two together first, then multiply that result into gl_Vertex (which is acting as a column matrix in this operation). To see this for yourself, try multiplying them the original way, then do it the way you're trying to do it CPU-side, and print the results to the console. You'll see different results. Doing it by hand may also help realize what's going on.

Try:

vec4 pos = gl_ModelViewMatrix * gl_Vertex;
gl_Position = gl_ProjectionMatrix * vec4(pos.xyz, 1.0);

Thats the only thing I can think of is something with the w component. Regardless if you need:

vec4 pos = gl_ModelViewMatrix * gl_Vertex;
//Do some calculations with pos variable
gl_Position = gl_ProjectionMatrix * pos;

You would have the same exact amount of lines of code by doing:

vec4 pos = gl_ModelViewMatrix * gl_Vertex;
//Do some calculations with pos variable
gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;

so just do that anyway.

Trying to break this very common line of code:
gl_Position = gl_ProjectionMatrix * gl_ModelViewMatrix * gl_Vertex;into vec4 pos = gl_ModelViewMatrix * gl_Vertex;
gl_Position = gl_ProjectionMatrix * pos;

[quote name='rubenhak' timestamp='1315637398' post='4859902']Trying to break this very common line of code:
gl_Position = gl_ProjectionMatrix * gl_ModelViewMatrix * gl_Vertex;into vec4 pos = gl_ModelViewMatrix * gl_Vertex;
gl_Position = gl_ProjectionMatrix * pos;

This looks very weird, as matrix multiplication operation is associative (A * B) * C = A * (B * C).

[quote name='rubenhak' timestamp='1315637398' post='4859902']This looks very weird, as matrix multiplication operation is associative (A * B) * C = A * (B * C).