Nucleo Pro 3 for After Effects CS5 has been released, and includes Spec-Preview, which continuously fills in your AE RAM cache for rapid on-demand views of your compositions. It also:
:..includes a slew of powerful timesavers, including our industry-acclaimed Background Render Queue.As the name implies, you can continue working in After Effects while Nucelo Pro 3 renders your compositions in the background at lightning speed. But faster rendering is just the beginning. Nucleo Pro 3 includes a wealth of fantastic workflow enhancements that will help optimize your time and customize the way you work."
Sounds like it should be built right into After Effects!
Update: The product has been discontinued, but Steve Forde: new product manager for After Effects helped create it.