a new operating system, new requirements for encryption. so you have snow leopard and you still want your gpg goodness. it's pretty easy - and you only need three bits of software. download these three things:
install macgpg - this is the backend framework for all your gpg action. now if you've already created your gpg keys you don't need GPG Keychain - you can check by looking for your .gpugpg folder in your home directory. if you don't have that, run GPG Keychain and it will prompt you to create your public private pair. make sure you pick a good strong password. you can always use gibson's password generator.
you're almost there, now you just need the mail plugin, that was your third download. just drop that in ~/Library/Mail/Bundles/ and you should be good to go. if you never activated plugins for mail before you need to active that - you can use the terminal or just use the Secrets application. restart mail and you're done.
for extra geek points you can import new keys by:
> gpg --import friends_key.gpg
isn't encryption the greatest?