It has been designed to be largely compatible with existing code. Also it will be quite a while before PHP5 becomes the default (or even common) version installed by most ISP's. You can see how minimal the compatibility issues are at the following link.
What I'd rather hear is "I've tested Moodle thoroughly with PHP5 and these are the things that need to be fixed (sample code included)" 
Yes, I hearded about the released I tested it on my production server it seems fine... but not all the features has been tested yet...
Hope to here from others...