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Probably not happening anymore with Debian Etch as stable, but that error happened sometimes when apt-get update:
W: GPG error: http://security.debian.org etch/updates Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A70DAF536070D3A1
W: GPG error: ftp://ftp.hr.debian.org testing Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A70DAF536070D3A1
W: You may want to run apt-get update to correct these problems
I've solved it just with:
apt-get install debian-archive-keyring
Source: LinuxQuestions