You just have to be a member by December 2006 -
"You do not have to be a member to do so, but this will ensure you receive competition details early.
The official launch of the competition will take place in December 2006 at which time you will need to be a member of the Royal Aeronautical Society."
Plenty of time though!....
(Edited to reflect in the excitement I didn't read the compo rules properly - do'h!)