single instruction multiple data

SIMD is processor architecture. C# is a language that is compiled for a virtual "universal processor" (the CIL, or MSIL), which is then again optimised to run on the physical processor. As such, C# is very remote from the physical processor's architecture. If C# was to take advantage of SIMD architectures, this would not be impossible, but it would be an effect of the porting of the CIL to an SIMD architecture, and of optimisations that lay way beyong the reach of C#. The latest incarnation of C# (4.0) already contain various optimisation possiblities towards parallelisation, but NOT optimisations towards SIMD architectures. If you want to look at code for SIMD architectures, you will have to (today) look at very much lower languages, and to my knowledge, at the pure assembly language (for example the instructions for Intel's MMX instructions). Higher level-languages, like C#, or C, may attempt to profit from these instructions, but this will always be through their compilers attempt at optimisation. Now, there may be higher-level languages that offer you the option of targeting these architectures, but you are in the area of fundamental research here. I suggest you orient your query to more 'academic' forum.
Oh, yes and BTW, SIMD cannot be used to optimise 'any' code. The application area of SIMD is quite narrow, and mostly applies to Vector and Matrix operations, which makes it very well suited to graphics and visualisation.