A group of developers of the community have created a very good tutorial about the Joomla's MVC (model, view, controller). The mission of the Model View Controller Component Tutorial Collaboration (MVCCTC) is to create documentation describing how to create components for Joomla! 1.5 using the MVC pattern and other Joomla! 1.5 libraries.The MVCCTC promotes the production of complete and well formed Open Source components for the Joomla! 1.5

This tool could be very useful for every developers and it won't be surprising to see many good 1.5 extensions coming from it! 

You can access to the tutorial here :)