Although many 32 bit programs will run in Vista 64 bit systems, the shell extensions (context menu items) will not be available unless they are specifically written for 64 bit.
At least this is my understanding after doing some additional research.
I like to write my own shell extensions. I don't have a 64 bit OS to test them but it's
my understanding that unless they are compiled with a 64 bit compiler, the only way
to use them with a 64 bit OS is to run a 32 bit shell. My next PC will most likely be running 64 bit Vista Home Premium so I guess I'll find out then.