Even I was getting this error "W: GPG error: http://nginx.org lucid Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY ABF5BD827BD9BF62", so I started searching and landed here.

And Yes, that mail thread reply really worked for me.

$ wget http://nginx.org/packages/keys/nginx_signing.key

$ cat nginx_signing.key | sudo apt-key add -
OK

$ apt-get update

$apt-get install nginx

It install version 1.4 which is latest I guess.

Thanks,
Prashant