There are a couple of discussions going on about unenrolled Baha'i identity: One is on talisman9, where proofs of the growth of unenrolleds are being discussed. (You have to join to read it.) The other is Steve Marshall's blog article "No Assembly Required". Properly speaking, Steve is an inactive Baha'i, but he shares some of the same perspectives and motivations that ex-members have -- and, of course, his wife Alison is unenrolled, forcibly taken off the rolls by order of the House. (On second thought, "inactive" seems an inadequate description of the editor of Baha'is Online. We need a better word for an enrolled Baha'i that doesn't partipate in administrative matters, but is very involved with the Faith otherwise.)
I'm extremely shy of estimating numbers of unenrolled Baha'is -- I think that being unenrolled, to some extent, means getting away from the obsession with statistics that characterize the administrative Faith. However, I read one Baha'i scholar, who once worked at the National center, estimate the number of unenrolleds as around 10,000 in the U.S. -- and this included people who had a dual religious identity. He said that polls he participated in turned up Baha'is that National never heard of i.e. people who identified themselves as Baha'i but had never registered. But that number is still just an educated guess -- that's probably all we'll ever have.
What I see happening is that the *idea* of being an unenrolled Baha'i is growing -- people that at one time would have thought of themselves as ex-Baha'is who still retained an appreciation for Baha'i ideals are realizing that being off the membership rolls does not have to mean an abandonment of religious identity. Likewise, there are enrolled Baha'is who accept unenrolleds as fellow believers.
Sen McGlinn argues that this is a positive development in the religion -- that it marks an emergence from a sect-like structure. Think of any "cult" group you can name, and there is no distinction at all between membership and adherence, whereas one can run into Christians, Muslims, or Buddhists who do not belong to any formal organization. That is, having unattached adherents is a sign of maturity in a religion, not a sign of growing opposition, as it is sometimes described.