"closed-port" backdoors

[Andreas Hasenack <andreas () CONECTIVA COM BR>]

Has somebody seen in the wild a type of backdoor where
no ports are open until a specifig set of packets are sent
to the machine?
For example, the backdoor would only bind to port X if
the machine receives SYN packets to three other ports in
sequence. I've seen code to do this (and sorry if it's not
new), but I haven't seen rootkits using it.