Update: Oh! And since even I missed this link in the original blog post, you can request to be invited. Not sure if that results in an email, either, though.
Update #2: Also, we're trying out Vanilla forums as an alternative to the delicious mailing list. If you're invited to the preview, your login should work there too. We might even launch public forums once del 2 is out.
In case anyone's forgotten: I work on delicious. Since I've had a steady stream of friends and acquaintances asking me about how they can get into the invite, I thought I'd mention this:
You may already have been invited! :)
So, the problem is that we have many more people invited to the preview than have actually started using it. As briefly mentioned in the blog post announcing the preview, you need to check your "Links for You" to see if you've been sent a link to the preview.
But, if your invite has possibly been buried in other links from friends - you might just try signing into the preview with the username and password from your existing delicious account! It might just work.