Apple has confirmed that Leopard upgraders might encounter a few teething problems, saying "After completing an upgrade installation of Leopard and restarting the computer, a 'blue screen' may appear for an extended period of time", adding that "It may be necessary to perform an Archive and Install installation of Leopard".
Early adopters of the new Mac OS X operating system have reported several instances of these Blue Screen problems which have hitherto been a familiar sight for Microsoft Windows Users.
A Computerworld article points quarely to the Unsality's APE software as the root of all evil.
This module allows Mac Users to change the look and feel of the Operating System but apparently hasn't been upgraded in the last iteration of Mac OS X, making it unstable when running the new Intel Processors.
Upgrading to the latest version of APE should solve the issue.
Unlike Windows users, most Apple users upgrade their operating systems rather than install a fresh copy of their OSes on a clean computer.