This is a very good idea Richard
Personnally I think resistance to opensource will more likely come from corporations, not SMEs (at least, those I sell opensource CMS don’t have such concerns). They (the big Cs) tend to lock too many things under a tight leash, and are more often than not staying in the field of "business-as-usual" to paraphrase the dear Cluetrain manifesto... But let’s get back on track with your idea...
First, the thing you’ll need, to start a gallery, is not only enough MODx based corporate websites (at least 10, possibly more, this can be discussed...), but also websites with nice & clean designs, strict standards compliancy would be a definite plus. What would be nice is for people who have done / are building corporate websites to share with the community and announce them here.
Second, your idea gives me another one : build a MODx websites gallery grouped by category (corportate, blogs, webzines, communities...and so on). That would not only advertise MODx as being a credible platform for corporate websites but make a point about its versatility : as I said before to me MODx is a tool enabling you to build websites based on your users/clients needs and NOT from a locked, pre-defined use (blog, portal, news...).
Right now I think we should start by building a list, inviting people to share their MODx website adress (which I guess is the point of this forum), and prepare for a future gallery.
I for one will certainly use MODx for corporate projects, but I expect to have something tangible (a working corporate website) by the beginning of 2006.