Although Softimage is great and all Maya is better in most cases. The fact that they used Softimage instead of XSI is because they have used it for ages, when Soft was the only good software for character animations... They had all kinds of special software written for it too... It's not easy to change software when people have been using it for ages. The proof is that they didn't change to XSI even if it has been out for over 3 years.

But except for the character part most of the other stuff is done in Maya. I saw a 1 hour seminar at the 3d festival when Sean Shur from ILM talked about the making of Episode 1 and most stuff was done on Maya. For the space battles they used Electric Image and for the characters like Jar Jar they used SoftImage. But all the rest, including the WHOLE Podracing sequence Tolerate, they used Maya!