Really appreciate your effort! It is good (and rare) to see a controled test. Are these pictures taken under jpg mode? It looks to me XTi is using different curve & is prob. less sensitive. Have you tried to shoot in Raw indoor with fix light source? With raw & indoor, you can play with curve and match XTi's to XT's and then compare noise @ ISO1600. Love to see it if you plan to do more tests... =)