last time we heard from the development team in charge of upcoming indie sandbox post-apocalyptic MMO, Xsyon, they had planned to let players in prior to launch in small groups. The primary reason was to allow the developers enough time to test, then let another group in, and repeat. Staggering the pre-order invitations to ensure they had bugs fixed sounded like a wise move to us.
Now, in the most recent posting to that thread, the current plans are to let the remaining players in to get their hands on Xsyon starting this upcoming Wednesday. From there, they'll focus on fixing the last of the bugs and hopefully testing for two weeks. At that time, they intend to do the final wipe and then let players have their pre-order week.