Joomla 1.5.x is a much different animal than Joomla 1.0x - the install does not work exactly like it used to so I noticed one thing that needs to be carefully done during the install or support calls go through the roof on this script.
The FTP settings for the Joomla Administration panel need to be turned on and need to be set to the website's FTP user login details and the FTP path has to be set to the account's website default folder where you plan to install Joomla - example "/public_html" or "/domains/yourwebsite.com/public_html".
The install instructions aren't clear enough on this and leave the novice user guessing what they need to be. Once I went through several support calls on this I sat down and figured out exactly what was happening and what had to happen in order for the novices to be able to successfully add templates, components and modules to the Joomla site.
If all of this is done properly then all files for the Joomla script and extensions will be owned by the site owner/username instead of having to tweak the site folder permissions for templates/components/modules to apache.apache like we used to. Now if the FTP settings are done correctly in Joomla you never have to worry about this anymore. Without doing this correctly all of these extensions won't install properly and will show lots of permission errors during their install.
You will have to set the /tmp and /cache folder to 777 write permissions as well.