In the press release, it does say "Further discretionary selections will be announced later this week."
For this year's nomination information, it can be found at:
http://www.usacycling.org/forms/2006 MTB Worlds Athlete Nomination.pdf
For this year's nomination information, it can be found at:
http://www.usacycling.org/forms/2006 MTB Worlds Athlete Nomination.pdf