As for traffic, we initially got retweeted a couple times by people with a few thousand followers in the design space. Both products also were on news.layervault.com as a result for some time, as well as BetaList a bit later on.
Our conversion rate to beta signups is around 30%. We've had nowhere near 10,000 or even 100,000 visits.
I wish more blogs would include the "luck" details in their success posts. It's practically lying to say that all that happened was that you tweeted to ~100 followers. I'm not trying to criticize, it's just an interesting bit that makes the startup sound much more down-to-earth when they're willing to admit where they got their luck from.
Because let's not forget: all success is due to some luck, and that's sometimes the most interesting part of the story.
I'm familiar with the lean startup gospel. I find it too neat; it's too close to the protestant work ethic to be believable. I'd like to hear more about the "luck", grease, handshakes and other details that don't fit into the usual startup narrative.
It was much easier than we thought it would be.
I would say that we could've driven more traffic to the site and increased the absolute conversions while lowering the conversion rate BUT I'm comfortable with having our beta site a little more hidden than say a general marketing splash page.
My two cents are that if you have a compelling product, your beta launch page is a pretty trivial commitment and you should expect conversion rates much higher than Ecommerce. 30% is not unreasonable if you're not spamming. If you're spamming, 10% is achievable but hard.
Traffic sources: Our Portal, Twitter, Launchrock virality. I have to say the Launchrock virality techniques worked much better than I thought they would. It made my job as a marketer much easier.