Uruhara told Ichigo way back when he started to train him that he had been using Rukia's shinigami powers until that point. Since those were removed, he'd have to bring out his own shinigami powers. If Ichigo's powers were Zangetsu, then it makes sense that Rukia's powers were Sode no Shirayuki. This is more implied that confirm though, so it would be good to get some proof.
Fortunately there should be a way to prove this. I'd appreciate it though if anyone has a picture of Rukia's zanpaktou in unreleased form and a picture of Ichigo's original zanpaktou so we can compare them. I think it should be possible to find both in episode 1. What we most need to see is the hilt design.