First, we will be culling "dead" users (accounts with zero posts), and will continue to do so as part of regular maintenance. We apologize if your account gets deleted as part of this process — please simply re-register if this happens.
Your best bet is to just post, and thus keep your account alive.
Second, accounts that remain inactive (i.e. the email activation link was never used) for over one month will be deleted. Again, we apologize if this happens to you. Email does get lost sometimes.
These changes have been reflected in the New User's Guide.
Also the forum software has been updated, and some of the styling got hosed, so things are a bit uglier than usual — the gnomes are working on it.