That apparently was errata'd out with the rules compendium (I think?), so you can now use any implement you are proficient with with any power. This was done so that a Cleric/Wizard or Infoker|Wizard/Shaman wouldn't need to carry two/three implements.
While I can't find the exact quote from the book, see here: http://www.enworld.org/forum/showthr...35#post5483735