Additional info...
I would wait with upgrading any current joomla 1.5 site to 1.6 for a while. I just tried to find some upgrade instructions but mostly I find that the instructions to do it has not been written/published.
There is a component that can do it, but I would think that templates etc may break, so if you try it, make sure to have a copy of both your current installed joomla sites files and the database if things go wrong.