We've wanted to be transparent with the community on our plans for ArduPilot Mega to get feedback, but I fear we've also introduced uncertainty and confusion. Here are some tips and pointers that may help you plan your ArduPilot path.
First, don't expect ArduPilot Mega until well into the new year. Second, it will be considerably more expensive than ArduPilot. Third, it won't necessarily work any better than the current ArduPilot for basic autopilot functions--it's just for people who want two-way telemetry and a more compact installation. Fourth, we'll continue to support the current ArduPilot. These are two different products, for two different customer classes.
We'll also be releasing a product that's for a lower end of the market, an even simpler, cheaper autopilot for inherently stable aircraft (this is a turn-rate limiter autopilot that will outperform the PicoPilot at less than 1/4 the cost). Also Q1.
So think of ArduPilot as a family of products: Entry, General and Pro.
ArduIMU is not part of the ArduPilot line. It's for people who want to integrate an IMU/AHRS into their own project.The IMU in ArduPilot Mega will be custom for that product.
Unless you're really trying to save money, I suggest most people start with the current ArduPilot. It's mature, stable and will be supported as long as we can see. ArduPilot Mega will someday get there, but unless you're willing to deal with our teething pains for the next year, you'll be a lot happier with the current product.