I'm having this exact problem now.  Based on another post in this forum I deleted the database, set it up for a fresh install and reinstalled.  I got the same success message but in reading carefully I noticed that the portals failed to install.  In my case there is obviously a premissions problem in the file system because it is reporting access denied to ...\portals\0.  I'm still working but you should be aware that some install errors do not produce a failure message.

I had the same NullReferenceException when trying to install yesterday. Took me almost all day to figure it out. Looks like there is a problem in the Config.vb code. It uses an XPath expression to attempt to find the machineKey element and it doesn't seem to work all the time. I hard coded the selection path within the Config.UpdateMachineKey call to get past it. Mine is now:

xmlConfig.FirstChild.ChildNodes(5).FirstChild

instead of:

xmlConfig.SelectSingleNode("configuration/system.web/machineKey")

This should work as long as you are using a fresh web.config that was renamed from release.config. Otherwise you may need to play with the node indices to get it correct. FWIW I tried different XPaths that should have worked, but didn't for some reason and I didn't want to take any more time to figure out why.

After compiling this change and copying the newly compiled DotNetNuke.dll to the bin directory of my site I tried running the install again and it seemed to get caught up in some kind of loop. So to get past this I manually did what the code was trying to do and that is to add an "InstallationDate" element to the appSettings block in the web.config like so:

<add key="InstallationDate" value="5/1/2006"/>

Then I ran the install URL again and voila!