First off, a new GUI could very well be a new way to use your computer. What if they choose to go with a 3d desktop, where you had to navigate through a 3d space? What if the new GUI is using Core Animation -- that something Shapeshifer couldn't do without Leopard.
Second, I don't think Leopard is coming in Spring. It really can't. In June there will be some sort of conference thats all about Leopard. Does it make since to have that after the launch, when you barely even spoke about your new OS?
No, just like Apple TV that was going to be launched in February -- and according to Apple, it did -- but we all know it secretly without Apple's permission snuck into March. And, just like Leopard's brother, Apple TV (which more then likely uses Leopard) -- Leopard will ship later then June.
Question is, how late?
What If There Are No Top Secret Features in Leopard?