Which operating systems besides Ubuntu use it?
Upstart is the replacement for the traditional init.d style System-V bootup scripts. However, upstart is more than just a collection of bootup scripts. It allows in fact a minute planning and control of the start of different daemons. For instance, in order to automount network drives, you need first a working network. While before upstart these situations often led to race conditions, in the upstart declaration the prerequisite of a running network can be included.
Upstart is in fact based on an event monitoring system. When a certain hardware condition occurs, or another process sends an event, one or more of upstarts scripts might be triggered. This allows i.e. particular actions being automatically triggered when an usb stick is inserted or removed.
I believe all major Linux distributions are migrating System-V init to upstart step-by-step. In fact, upstarts can also trigger the start of the traditional init scripts, therefore, the transition does not have to occur all at once.
Upstart is an event-based replacement for the /sbin/init daemon which handles starting of tasks and services during boot, stopping them during shutdown and supervising them while the system is running.
The SysV boot process is strictly synchronous. Things happen one at a time, blocking future tasks until the current one has completed. If anything in the boot process takes a long time, everything else has to wait. Additionally tasks only run when the init daemon changes state (such as when the machine is powered on or off).
Upstart is a new init daemon that allows services to be started in response to events rather than in bulk runlevels. With each job file in the /etc/init directory being responsible for launching a service or for a specific component of system initialisation. There is no fixed sequence; instead each job specifies the events to which it will react. When an event occurs, Upstart starts all jobs that have been waiting for this event, in parallel.
You can theoretically use it even after the system is up and running. Upstart is eventually slated to take over tasks such as or plugging in external devices like thumb drives (currently handled by udev and hal), or running programs at specific times (currently handled by cron).
Upstart was originally developed for the Ubuntu distribution, but is intended to be suitable for deployment in all Linux distributions as a replacement for the venerable System-V init.
Upstart is a modern init replacement, http://upstart.ubuntu.com/ and is event driven.
Current users are Ubuntu, Fedora, OpenSUSE, Maemo, Palm's WebOS.