Leopard upgrade failed for me on a G5 due to a divX quicktime component - finder just wouldn't come up until I logged in across ssh and removed the offending file.

It is pretty unlikely from your description of the symptoms that you have the same problem though but thought I would mention it just in case.

see this thread
http://forums.macnn.com/90/mac-os-x/352097/finder-dead-after-leopard-install/