Trust Packets

Hello,

the implementation of Trust Packets is implementation specific.
Does anybody know, how these packets are implemented in PGP 7.x or
higher?

BTW, is there a reason why they are implementation specific? If somebody
uses
different implementations (e.g. on different platforms) of PGP, he will
not be
able to use his "one and only" keyring.

Thanks in advance,

Thorsten