Leopard and Tiger networking: why won't this work?

Ok, this seems to be a really strange problem, and chances are I'm just doing something stupid. I have an iMac running Tiger, and a Macbook which I just upgraded to Leopard. When they were both running Tiger, I could use the Go menu on the Macbook to connect to the iMac and browse it's files no problem.
Now, I can see the iMac from the Go menu, but every time I try to connect, it says "Connection Failed."
Any ideas as to what I'm doing wrong? I haven't changed any settings on the iMac since the install of Leopard, so I would assume security wise it should still just work.
I know this is something simple guys, any help would be greatly appreciated. THanks.