W: GPG error: http://nginx.org squeeze Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY ABF5BD827BD9BF62

If you get that error, add nginx public key to your keyring like this:

curl http://nginx.org/packages/keys/nginx_signing.key | sudo apt-key add -

...then you should be able to run `sudo apt-get update` on your newly added nginx repos.
This was on Debian 6, should work on ubuntu too.