It's highly unlikely that you'll ever be able to run Snow Leopard on your Nokia phone. The next best thing may be to install this Snow Leopard Inspired Theme on your device.
The theme adds a new background wallpaper to your home screen, featuring a vector image of a leopard peering out of a slit in the screen. The Snow Leopard Inspired Theme also applies a new blue color scheme to the Symbian interface, and adds a cool analog clock to your home screen.
Disappointingly, the Snow Leopard Inspired Theme doesn't alter any of the icons on your system. It would've been great to see some of those gorgeous Mac-style buttons added to the menus or shortcuts bar.
As it is, Snow Leopard Inspired Theme is a stylish, yet conservative Mac theme for your S60 3rd Edition phones.