Here's the official blog announcement - http://www.dotnetnuke.com/Community/Blogs/tabid/825/EntryId/2122/DotNetNuke-5-0-0-Released.aspx
And here's the full change release notes (way too much detail) - http://support.dotnetnuke.com/Main.aspx?ReturnUrl=%2fproject%2fChangeLog.aspx%3fPROJID%3d2&PROJID=2
So here's the highlight bullet points:
- There's new support for XHTML and CSS in skins - removing lots of the non CSS features (table dependent things) - skins should now be much easier to design - they should be less DNN specific.
- Beneath the covers there are more unit tests and more testability - which should lead to long term improvements and stability
- IE8 web slice support has been added
- The installer has been improved
- Admin pages are now no longer quite so special - they're now just normal pages with admin rights - also "deny" has been added as well as "allow" to all the permissions grids.