Actually, I think it's more simpler than a new reiatsu. I believe that Ichigo is on a whole new plan of power. Remember that Ichigo's dad (or Gin, I forget who) said that only those that were on the same level as Aizen understood his newfound power. Ichigo's dad, Urahara, and Yoruichi didn't understand it thus they weren't on the same level as Aizen; unlike Ichigo who was scared out of his mind. Fast forward to Deicide 19, no one senses reiatsu from him because he's on a total different level from those around him and for a small amount of time, Ichigo became the strongest being in the Soul Society therefore, Aizen doesn't sense reiatsu from Ichigo because Ichigo has surpassed him!