I've been trying to install the NetBeans 6.9 official release (netbeans-6.9-ml-java-macosx.dmg) but it kept on failing during the "Preparing Installation" phase. No detailed errors were shown other than a generic statement to contact my software vendor.
Turned out, at least on my MacBook Pro, that the cause seemed to be that XCode was running at the same time. As soon as I quit from XCode and ran the NetBeans installer from the same installation package, it worked without any problems and installed NetBeans successfully.
Don't know if this is isolated to NetBeans or more generally to the installer technology used on the Mac, but it seems that there was some contention with Xcode, which resulted in the installer not working.