Problem upgrading from 10.6.8

I'm trying to install Lion GM over my current SL installation, but once i got past the SLA and confirm the installation disk, dialog pops out asking for admin password. Once entered, it disappears and then... nothing happens (not even error message).

I tried all of my disks (the current one with SL, then empty partition and then even my external drive). None of them worked.

The only error I got (only shows up in Installer Log, no pop-up window or notification whatsoever) is "IATool failed to launch". I made a quick search but not found any reference of this error anywhere.

Anyone had this error as well? Any suggestions on how to resolve it?

Thanks!