If a widely-distributed MUA is requiring the text/plain
part to be manually opened, that needs to be checked
into. Maybe a 'Content-Disposition: inline' is needed
on the text/plain.
I'm using Outlook Express 5.5, with PGP 7.0.3 extensions installed. Both the
message text and the signature appear as (separate) attachments. I don't know
why, exactly. I've received signed messages from other PGP users, and they
appear normally.