Shader issue: Normal map from tangent to world space - Textures flipping?

I'm currently trying to write a snow shader. I'd like the snow to appear on top of a mesh, of course. The snow covered areas should also depend on the material's normal (bump) map so that it appears on top of the details provided by the normal map as well.

To achieve this I convert the normal map from tangent to world space in the pixel shader. The amount of snow then corresponds to the normal map's y component.

This works quite well so far except for one issue: When rotating the GameObject at certain angles the textures seem to flip so that the snow is no longer on top of the details provided by the normal map but beneath them.

Here's a video illustrating the issue (the snow is painted blue for testing purposes): http://www.youtube.com/watch?v=6VG-yoeqqWk. As you can see the blue snow at first appears on top of the rocks "sticking out" of the cube. But at 45° the textures flip (they flip again at 135°, 225° and 315°).

What could be wrong with my shader?

 Shader "SnowMaterial"
 {
     Properties 
     {
         _SnowColor("_SnowColor", Color) = (1,1,1,1)
         _SnowBump("_SnowBump", 2D) = "bump" {}
         _MainTex("Base (RGB) Gloss (A)", 2D) = "black" {}
         _BumpMap("Normalmap", 2D) = "bump" {}
     }
     
     SubShader 
     {
         Tags
         {
             "Queue" = "Geometry"
             "IgnoreProjector" = "False"
             "RenderType" = "Opaque"
         }
 
         Cull Back
         ZWrite On
         ZTest LEqual
         ColorMask RGBA
         Fog { }
 
         CGPROGRAM
             #pragma surface surf BlinnPhong vertex:vert
             #pragma target 3.0
 
             float4 _SnowColor;
             sampler2D _SnowBump;
             sampler2D _MainTex;
             sampler2D _BumpMap;
 
             struct Input
             {
                 float3 sWorldNormal;
                 float2 uv_SnowBump;
                 float2 uv_MainTex;
                 float2 uv_BumpMap;
                 float4 tangent;
             };
 
             void vert(inout appdata_full v, out Input o)
             {
                 v.normal = (float4( v.normal.x, v.normal.y, v.normal.z, 1.0)).xyz;
 
                 o.sWorldNormal = mul((float3x3)_Object2World, SCALED_NORMAL);
                 o.tangent = mul(v.tangent, _Object2World);
             }
 
             void surf(Input IN, inout SurfaceOutput o)
             {                
                 float4 mainTex = tex2D(_MainTex, IN.uv_MainTex);
                 float3 mainBump = UnpackNormal(tex2D(_BumpMap, IN.uv_BumpMap));
                 float3 snowBump = UnpackNormal(tex2D(_SnowBump, IN.uv_SnowBump));
                 
                 float3 tangent = normalize(IN.tangent.xyz);
                 float3 normal = normalize(IN.sWorldNormal);
                 float3 binormal = normalize(cross(normal, tangent) * IN.tangent.w);
                 
                 float3x3 tangentToWorld = transpose(float3x3(tangent, binormal, normal));
                 
                 float3 amount = saturate(mul(tangentToWorld, normalize(mainBump)));
                 o.Albedo = lerp(mainTex, _SnowColor, amount.y);
                 o.Normal = lerp(mainBump, snowBump, amount.y);
             }
         ENDCG
     }
     Fallback "Diffuse"
 }