After Ubuntu and Fedora, Debian also migrates to upstart.
To summarize the article in the previous link, the main reason for the change is that the kernel is now no more blocking on some hardware scans, and this might make some init scripts (e.g. fsck, network) start before the device is initialized. We now need an event-based init system, so the kernel could "tell" it when the device is ready. This is something that upstart is capable of.
Now I wonder if RHEL6 would inherit upstart from Fedora, this will mean a complete victory for upstart.