Right now it would be the top 25 in North America. The last place person to get an invite would be
25 Andrew Cox 1850.72 MA MA US
The big variable is Nationals. The top 4 get an invite, plus a trip. 5-8 get an invite. What happens if any (or all) of these 8 also end up in the top 25 in rankings?
It has been assumed that their invite will pass down, so potentially the player in 33rd place may actually end up with an invitation to play. However, I don't see anywhere on the FAQ for Worlds that this is correct. It may just be 25 players from North America.
http://www.go-pokemon.com/op/premierevents/championshipseries/worlds/info/07-08/answers.html