I found a reference to this event on Tom and Marna's miatafun.net site and found the announcement at miata.net. Dates are 23-26 May 2008

"We are pleased to announce MX5-XX, Miata World II in Dallas,
Texas, Memorial Day Weekend 2009 in celebration of the Twentieth Anniversary of the Miata. For those who attended Miata World ’99, you know what to expect. Miata World II will surpass ’99 in every way with more cars, more special events and some really big surprises! Mark your calendars now.
More details will follow soon! Thank you."

We'll keep you informed as new details emerge.