Another big change for OSD in SCCM 2007 is the introduction of the 'task sequencing' engine. As the name suggests, task sequences allow administrators to define a series of steps that are followed in order for installing an image - such steps include capturing machine and user state, defining additional programs to run after the OS has been installed, whether or not to reboot the machine during image install, manipulation of bitlocker settings, disk partitioning, etc. Once a task sequence is defined it is advertised to a collection of machines and will be installed either as an advertisement or through bare metal provisioning. BDD 2007 contains a version of the task sequencing engine used in SCCM 2007 so if you want to see a preview of how this works, take a look at BDD.
Task sequencing is a feature of OSD and is found under the OSD node of the administrator console but isn't limited to just working with OSD. Customers have asked for a long time for the ability to control the execution order of advertisements. Historically the answer to this was to use dependent programs - but this approach doesn't address every situation. The task sequence engine is the answer to this request and allows very specific and granular control over what executes and when it executes.