I just found a comment in an earlier posting on this subject from Tyler (thanks Tyler!) that raised a good point. He was commenting on the difference he noticed in the shutdown speed after upgrading from 2GB of RAM to 4GB.
Now by default the "new" (all intel MBP's I believe) MBP's create a sleepimage file by default as they are set to hibernate instead of sleep, so you could draw a conclusion from this that a 4GB file would take longer to create than that of a 2GB file but this would only make sense if you were sleeping the MBP, not I would have thought, shutting it down.
It also doesnt explain why, on a fresh install of Leopard, that my MBP shuts down just as quickly with 4GB as with 2GB of RAM, which can only mean that its something I have done in terms of a program I have installed.
Tylers point made me think though if the "quality" of the RAM could factor into this equation as well? Then again, am I just over complicating this?