signatures from revoked key, trusted?

[Gabriel Philippe]

Hi,

A friend sends me signed messages wich signature is said correct by
GnuPG: "good signature from...".

I have just noticed I had signed his key with my old key, which is now
revoked in my keyring. So why does GnuPG consider the signature
correct? I would expect that, since I have revoked my old key, all
certifications done with this key should not be trusted anymore.

-- 
Gabriel