Is it possible to get the current Object's 'base' world position in a vertex function?

I was thinking something like this:

float3 baseWorldPos = mul ( _Object2World, float4(0,0,0,0) ).xyz;

...but on testing I'm fairly certain it's returning a zero vector ( because its a multiplication? ).

Otherwise is there another way to get a distinct value per object, so that a shader's effects can be made unique for each individual object?

Thanks.