There's a nice, sort of generic Article on E-mail Newsletters over on ClickZ today. The emphasis is lightly on B-to-B.

It's interesting to see a ClickZ columnist apologizing for recommending double opt-in. Generally the tone is you absolutely, positively have to do it. Here it's more like: I know almost nobody does it anymore, but you should maybe sort of consider it. It's also interesting that she claims double opt-in only culls about 20% of subscribers. I would have expected that to be higher.