tbh, I haven't found a need for it. I came across it earlier cos I was looking for a fix for KOTOR not running on dual cores. Seems this problem is related to the dual core optimisations with the latest forceware 80xx series drivers. However, I digress...(and fixed that problem with a registry change)
Regarding the use of a switch in program shortcuts to set processor affinity, I fairly sure this isn't possible for all programs (ie. I don't believe there is a windows switch).
However, there are some programs that have a switch available (eg. folding program), but these have been written into the programs.
I realise we're going way off topic here, but just in case someone happens on this thread using a search, this is what I found.
Due to dual core optimisations in nVidia Forceware drivers, a number of games do not work well with dual core processors. nVidia then implemented a method of disabling these dual core optimisations in release 81.98. Unfortunately, it's a registry hack, and apparently not well documented in the release notes.
The following link explains how to make the necessary registry change, depending on whether the game is OpenGL or D3D.
To set the affinity of a program, under task manager, you can right click on an applicaton and go to process, or you can go directly to the processes, then right click on process and go to set affinity. From there, you can limit a program to a specific core.