Core 2 platform can support 16GB, however it was up to OEMs to enable it for higher density memory.
You could try a stick of 8GB in the 2009 if you're running Lion or higher as Apple enabled support for the 2010 model which uses the same memory controller, keep in mind once you cross past the 8GB mark booting back to a Snow Leopard can't be done. I'm guessing Apple held out 16GB support for a native 64-bit OS due to kernel limitations of SL.
I own a 2010 13" MBP, did it just to see what would happen and being unable to dual-boot Snow Leopard(for Rosetta) left me downgrading.