Discussed on roundup-users, raising as RFE for 
posterity:

Allow users without email addresses and do not 
attempt to send email to such users.

John Rouillard wants this for users who don't have 
email addresses; I like it for users who don't want to 
be sent Roundup email.

Richard suggested implementation by 
making "address" non-:required on the user 
registration and edit forms, and by tweaking nosy-
reactor etc to handle non-set addresses.

I thought this was useful and intuitive enough to be 
part of the default installation, hence this RFE.

(I'd link to the relevant thread in the roundup-users 
archives, but SourceForge seems to be playing up and 
I can't get to the archives -- will find the link tomorrow.)