I would kill for an email at Perl Monks. To address all of the issues that you have presented, AgentM
I think that all users over level three should get to use this feature. The reason I say this is because of the fact that many people register nicks and never come back. I don't think that we want this happening with email addresses.
I think forwarding is certainly adequate.
I believe that the user should be able to change the forwarding address. I see no reason why they should not be allowed to.
I HATE signature advertisements on emails. I would rather pay a monthly fee than deal with signature advertisements (and I would too).