The member list over at OpenXMLCommunity.org is definitely growing rapidly. I think they've already more than doubled the number of registered supporters since it started last month.
I recently saw this statement from HP about the standardization of Office document formats: http://h71028.www7.hp.com/enterprise/cache/514506-0-0-0-121.html
Hewlett-Packard company is in favor of standards for office document formats, including the Office Open XML (OOXML) and Open Document Format (ODF) standards. We believe that the two standards will co-exist interoperably, and that customers should have the opportunity to select the standards which best fit their needs.
In the current vote on OOXML at JTC 1, HP is supporting an affirmative vote in those national standards bodies in which HP is active.
I think it's clear to most that if ISO takes on the stewardship of the Open XML specs, it means that everyone will be able to access the full documentation on the file formats going forward (regardless of what may happen to any one company). Document archival is extremely important, and the approval of Open XML will mean that we can take all the legacy binary documents and move them into a format that is not just fully documented, but maintained by an independent body.