Upgrade from SQL 2000 to SQL 2008 is now officially supported. In the early release of SQL 08 (currently is RC0), we are not sure whether there will be an upgrade path from SQL 2000 to SQL 08. This uncertainty was over when I was installing RC0 last night. The option to upgrade from SQL 2000 instance is there.
Ok dude, the menu is there. What about the guidance? We know that there is more than 300 pages whitepaper available for SQL 2005 upgrade. But the one for SQL 08 is not available as of this writing, please be patient to wait. The Microsoft Official Curriculum for this topic is under development now, and I am also involved in it. The alpha run of this training will be held around September 08, and is expected to hit the market 2-3 months after that.
My advice for you that still stick with SQL 2000 is: don't upgrade to SQL 2005. Please wait until the release of SQL 08, and do direct upgrade to SQL 08 instead :)