Clare Redstone writes:
How insecure? Are they more vulnerable than a members-only Yahoo or Google group for example?
Probably a little more vulnerable, for social reasons. Your members- only password at Yahoo/Google is your personal mail password; people probably protect those fairly well (although often enough dinner guests can read them off the post-in on the monitory in the corner ;-). Mailman subscription passwords tend to be easier to guess.
Are they protected from search engines?
Yes.
Would someone have to make a deliberate effort to hack in to read the archive
Yes.
or could someone come across it by accident, say through a search engine?
No.
I think if it would take someone with some technical knowledge, deliberately looking for it to get in, that would be safe enough.
Stealing a password doesn't take technical knowledge, but it clearly takes evil intent.
One thing I'm suggesting is that people could set up and email account with a nickname so they wouldn't so easily be identified.
This is a good idea any time you want to preserve a modicum of privacy on the 'net.