Their website is confusing. Just as I was about to install, I wondered if I am installing the latest version or not?? I downloaded version Joomla! 1.0.13 Stable [ Sunglow ] because it was at the top of the page. However, at the bottom of the list is Joomla 1.5, which obviously 1.5 is a bigger number and should be the latest version, right? In their news they talk about version 1.5 just getting stable, released. Why then is the version 1.0.13 at the top of the list? I think it's a very confusing organization, because you'd usually want to put the latest version at the top. If anyone from Joomla is reading this, I suggest you change the order of your download list.
The reason is that it is at the bottom is because it is a release candidate. It is not yet ready to go live - it's still in testing phase. Download 1.0.13 and use that for now.
Some notes here: If you want to build a brand new site I suggest you take version 1.5 cause you won't be able to upgrade from 1.0.13 to 1.5.
from what I hear, they have changed many things on the Joomla core itself. Joomla 1.0.13 was derived from Mambo. That's why you won't be able to upgrade. 1.5 is almost completely different from 1.0.13. I don't know much about programming but 1.5 feels lighter and faster. It also supports SEF URLs out of the box. But don't expect for lots of extensions for now.
Yeah, the whole core has been rewritten and it's now more like Drupal in the way that it is a Framework first and foremost with a CMS built using that framework. As far as upgrading is concerned, the other posters are correct in that an 'upgrade' as such is not possible. There are components out there for 'core' content migration, though, and in reality most of the other components shouldn't have a problem themselves with their own data.