The link between ETC (see below) and Disney on the Mission Space project has been apparent since early on in the development of the new pavilion. ETC have recently launched a new website, which appears to give some major clues to the Mission Space ride system. Of course, this is not officially confirmed, and this information is largely based on rumor, and should be treated as such.
The link below is part of the new ETC Entertainment division http://www.therideworks.com, and appears to show concept art of the ride system that I discussed in the update below, dated 13 June 2001.
Notice the central centrifuge design, with a large number of Cyberspace Mountain type motion pods attached. This configuration exactly matches the rumored Mission Space ride system.
As a final teaser, here is a quote from their website...
"In 1999, Entertainment Technologies Corporation [EnTCo] was formed to focus ETC's unique experience and expertise toward creating the next generation of motion-based simulation attractions.
2001 saw the opening of The Ride Works, EnTCo's premiere attraction development facility. The Ride Works offers the world sustained G's and continuous 360º motion in up to four axes.
Wait till you see what happens in 2003 . . ."
The final line referencing Mission Space perhaps?