I'm using the tex2D function of HLSL, and I am wondering what will happen if I try to grab a pixel from a pixel coordinate outside of my texture (as an example 1.1). Will it clamp? Will it repeat the texture and grab something else? Will it return transparent?
Depends on how the texture is defined I suppose - I haven't used XNA, but textures (or texture stages) generally have options on whether to wrap, mirror or clamp.
In OpenGL, the wrap type is defined on the texture object, on DirectX it's set in texture stages. Both have their good and bad sides.
Common wrap types include: