Sampling Texture in Vertex Shader?

Hi, I'm trying to sample a texture in a vertex shader, but for some reason I'm always getting 0,0,0,0 back from tex2D.

For debug purposes I'm just forwarding the sampled data through to the pixel shader for display. The input texture is solid light green and can be sampled fine in the pixel shader. For some reason it's not working from the vertex shader.

My graphics card is an nVidia Quadro FX1500, which claims to support shader model 3.0 (which, I believe, is the minimum version for texture sampling from a vertex shader).

 Shader "VSTex"
 {
     Properties
     {
         _Tex( "Tex", 2D ) = "white" {}
     }
 
     SubShader
     {
         Tags
         {
             "Queue"="Transparent"
             "RenderType"="Transparent"
         }
 
         Pass
         {
             Blend SrcAlpha OneMinusSrcAlpha
 
             CGPROGRAM
                 #pragma target 3.0
                 #pragma glsl
                 #pragma vertex VS
                 #pragma fragment PS
 
                 sampler2D _Tex;
 
                 struct SVSInput
                 {
                     float4    vertex : POSITION;
                 };
 
                 struct SVSToPS
                 {
                     float4    pos : POSITION;
                     float4    color : COLOR;
                 };
 
                 SVSToPS VS( SVSInput input )
                 {
                     SVSToPS     output;
 
                     output.pos = mul( UNITY_MATRIX_MVP, float4( input.vertex.xyz, 1.0f ));
                     output.color = tex2D( _Tex, float2( 0.0f, 0.0f ));
 
                     return output;
                 }
 
                 float4 PS( SVSToPS input ) : COLOR
                 {
                     return float4( input.color.xyz, 0.5f );
                 }
             ENDCG
         }
     }
     Fallback Off
 }