According to the official lifecycle calendar at http://www.mysql.com/about/legal/lifecycle/#calendar, active support for MySQL 5.0 (including regular binary updates) will end on December 31st, 2009, which is about 3 weeks away.
Many folks are still using MySQL 5.0.45, as until October that was the package that came with RedHat. That was released in July 2007, over 2 years ago!
Upgrading to MySQL 5.1 is not difficult, though it requires more steps than just upgrading the packages.
There is a list with all the changes made that might affect the upgrade process at http://www.pythian.com/news/1414/new-in-mysql-51-sheeris-presentation/. This includes which variable names have been deprecated and changed, as well as how to upgrade stored procedures, functions, triggers and views so they work properly in MySQL 5.1.