I've set Membership up as desired, now using 2.0.3. I set a single communication to go out, it is activated, set for 0 days after subscription paid, and it shows up as "signup message" -- the only subscription is free, i'm using paypal single subscription gateway. No joy. I've tried with the free gateway also, no difference.
I checked the table m_communications, which seems to be fine:
id = 1
subject = Welcome
message = Greetings...
periodunit = 0
periodtype = d
periodprepost = post
active = 1
lastupdated = 2011-05-07 15:54:12
periodstamp = 0
I get no email for this -- I do get the standard WP email with user/pwd and admin note of a new user.