   persistent data with a vbo and vertex shader?
  i''m new to vertex shaders, and was wondering if vertex shaders (i.e. HLSL, Cg) support changing data in the vbo and having it be persistent.  i imagine it works like this:  all verts are fed to the vert shader and then on to the rest of the pipeline, so my naivette would tell me that any properties of a vbo element that the shader changes won''t be reflected in the actual vbo memory.for example, my vertex struct would look like:...float4 position;float4 lastposition;...and my vertex program would look like:...float4 newposition = //calculate postion//lastposition = positionposition = newposition...is this sort of thing possible, maybe even with a performance hit? 
  I think you''ll find its a one way pipeline (in hardware anyways, if you felt like writing your own software system you could do what you like), vertex data goes in one end, transformed by the program and comes out the other end not heading back to the VBO ram at all ;  thats what i was afraid of.  thanks for the quick reply. 