The ride system is basically a centrifuge, which simulates increased g-forces, and causes motion sicknes for many, many people. If you experience motion sickness at all, this ride is not for you, and if you are not prone to motion sickness, you may want to wait at least an hour after eating. Increased g-forces can also cause problems for people with high blood pressure. This experience will be very similar to a test performed on actual astronauts during their training (a test that sometimes eliminates people from the program).
As far as opening delays, the plan from the get-go has been to avoid a Test Track-like delay (2 years when all was said and done), so I don't expect anything that extreme. The official info given in cast promo materials was initially spring '03, then fall '03, and now it's back to spring '03.
BTW, Test Track was not the longest delayed attraction opening in Disney history. The winner of that contest is the Haunted Mansion at
Disneyland. The building sat empty for nearly 9 years before the ride finally opened to guests.
---Ooooh, I nearly forgot to answer the other question. The cabins each have a motion-base, so in addition to the lateral g-forces, you get some pitch and yaw as well (no, the cabins DO NOT turn upside down, forget what you may have read about that). Slight adjustments to angle, acceleration, and top-speed can have an affect (apparently) on how stable tummies will remain during the ride.
Mrs. WDW, it very well may be open by the time you visit in August, just don't hinge your enjoyment on that experience, and if it's open it'll be gravy (as long as you're not a quesy type)