Update - 2/9/15: Since publishing this blog we made a decision to push one aspect of this change to CU9 to allow some partners a little more time to adapt to this new behavior. The new signout.aspx code is still shipping in CU8, but we are not changing to the new logoff behavior, that is sending the user logging off to signout.aspx until CU9. To be clear, CU8 will actually contain the new signout.aspx page, but the code will continue to send users logging off to logon.aspx.
So all references to CU8 in the post below have been changed to CU9. All references to CU9 changed to CU10, and so on. Hopefully that’s not too confusing.
Now just between us, if you really want to take advantage of this new code in CU8 you can! As the code snippet below indicates (in which we have also corrected a small mistake), you can remark the line in web.config to ‘LegacyLogOff’ (or indeed set it to any value except “LegacyLogOff”, even removing the line completely has the same effect) and you will send users logging off to signout.aspx. Just be careful when you make these changes, and remember the upgrade order comments further down, they all still apply. Here’s a small table to make this all a little easier to grasp: