Die Lösung gibt's hier:
http://stackoverflow.com/questions/1102…h-wordpress-3-4
Ich hab's ausprobiert: funktioniert.
Well we figured it out. Apparently from 3.4 the "Template Name"-comment needs to look like this:
/*
Template Name: MyCustomTemplate
*/
And it won't show up in the admin if it looks like this:
/* Template Name: MyCustomTemplate */
Hope this helps somebody else.
Edit: "Wordpress" emailed us and said this is a bug that will be fixed with the next version.