On cyberlot's blog today, he mentions some additional backwards compatibility bugs that PHP 5.1 and 4.4.1 are adding into the mix.

As of PHP 5.1 and in seems 4.4.1 are adding another BC break one that could break anyone using a object based database library within there custom session handler, and also make any registered shutdown function useless if it relys on objects, although the bugs focus on the session aspect of things.

This isn't something you can just "Document" away like they are trying to do.

Basically they are saying any user made or PHP made object is useless during the shutdown phase of PHP.

I could definitely see this issue causing some headaches for a lot of people out there. For more information on this issue, check out the bug report he refers to...