It sounds pretty interesting, from this page on the Gentoo forums:
Upstart brings the concept of an event-driven init system, taking benefit from hotplug to decide when to start processes. Eventualy upstart should even replace crond and inetd (InitNG has similar plans to cron). Besides a dependency awareness (like the current gentoo initscripts), parallel startup (idem) and better hardware utilization (like InitNG and others), Upstart is supposed to bring response to ACPI events, avaliability of network connections and so on.
Here are some getting started docs which has some more specifics.