Looks like America votes for the finalists, plus each judge's one wildcard spot. The Joe's Place American Idol blog links to a website called bumpshack.com, where I found the following answer:
Originally Posted by Harmony2000
The tenth season of American Idol gives the judges, Steven Tyler, Jennifer Lopez and Randy Jackson, a bit more power when it comes to choose the finalists who will fight for the Idol title.
The judges will be responsible for the auditions until they find their Top 20, which will include 10 boys and 10 girls. At the moment, the judges already have their Top 40, but the Top 20 Idol wannabes haven’t been chosen yet to prevent names from being leaked too early.
The public at home will vote for their favorite(s) in the Top 20. The five most voted boys and five most voted girls will go straight to the finals. Each of the judges will have a very tough decision to make before handling a “wild card” to another Idol hopeful.
In the big finale, 13 lucky contestants will battle on stage for the title of new American Idol.
That was written a while back. Now that it's a top 24, maybe it will be 6 boys + 6 girls + 1 wildcard per judge = 15 finalists? It may even be top 15 regardless of gender. We might have ten women and five men in the finals.