I just upgraded to 4.3 (Win7 host, Sandy Bridge Core i7 processor), and VirtualBox is now complaining that I've got more virtual cores (4) assigned to my guest than I have physical cores (2) on my host.
While this is true, this wasn't an issue with 4.2. It still lets me run it, but it's not happy about it.
EDIT: Further investigation shows that VirtualBox docs have always said this was a bad thing to do, but didn't used to throw up a warning when you did it anyways.
The big releases tend to have a way of introducing bugs, that although get fixed, can wreck your day if you depend on it.