Alter-G is the company said to be behind the "zero gravity" trainer that Paula Radcliffe used before her Beijing debacle - not that it had anything to do with her poor performance, but it was probably the only thing she could use while recovering from her injuries. At least she was able to finish.

No matter whether you use Alter-G's equipment, ellipticals, or pool running, there are only certain components of your running form (i.e.:aerobics) that will be preserved. There is unfortunately no substitute for gravity when it comes to preparing your body for the merciless pounding of road racing. Any astronaut who has spent much time in space can barely walk on return to 1G, let alone the 3g of running. On the other hand, it will give you a leg up (so to speak) on the rest of your training when you return to the road.

Congrats on your recovery as you return to competition. I think you made a fine choice and are in good care.

A discussion of the Alter-G and Paula...

http://www.sportsscientists.com/2008/08/beijing-womens-marathon-preview.html