At what point does an e-mail list become marketable? Is their a
critical mass to pique the interest of an advertising network, a
national advertiser, a local advertiser. Think specifically about an
e-mail newsletter that informs shoppers about health & beauty related
issues.

Here's what I'm looking for:
*A summary and opinion of the research
*A list to many articles discussing the value of an e-mail list and critical mass
*Quality sources: Nothing from Wilson Web. Look for things like
marketingsherpa.com, clickz, mediapost, adage--thinks I can quote and
will be taken seriously.

In the end, I want to know the number: is it 25k, 50k, 100k users? 

Regards,
Zachary