I have just added a new section (approved by Gerald) to the bottom of http://gcc.gnu.org/gcc-4.6/changes.html
Its intention is to mention noteworthy internal changes, i.e. changes interesting for, say, maintainers of backends/frontends outside the tree, and of course plugin developers upgrading from 4.5 to 4.6. So please feel free to add entries there concerning the areas you maintain. Thanks, -- Laurynas