The problem of adapting to large scale usage affects the amount of data as well as the number of users. I don't know how well the volume information and the protection data, which are managed fairly simply, will stand up in a cell which has thousands of volumes. I don't think Coda's protection and volume databases will work well for very large cells in their present form. But I suspect the larger problem lies in administering, backing up, and restoring all that data, and not in Coda's internal functions.
Also, I have said that Coda is not suited to holding really sensitive data, because of its limited security at present; that is, because its internal encryption routines are crippled, and because things may remain in a client's cache for a long time. Moreover, Coda's constituent parts have not had a serious security review, or been exposed yet to systematic attack or exploitation attempts. On the other hand, you should probably not serve secure data using NFS or any other shared file system. Of course, you can always use encrypted loopback on Coda files on a Linux box.