I managed to solve my issue with the installer hanging in 10.5. I turned off Internet Sharing (I had my WiFi setup to share out the ethernet connection). As soon as that was turned off, the installtion preflight went very quickly. For anyone who's intersted, you can tell where the installer is stuck by looking at the Activity Monitor - put it in the heirachial view - and look at the processes started by the installer. You'll see that "Internet Sharing" is listed (the actually binary is in /user/libexec/InternetSharing"). I couldn't tell (by looking at the preflight script) why they were calling this program, and I could get it to run via the command line myself... but turning off that feature in the Sharing preference pane and then restarting the installer fixed the issue.