As seen here

Short version for the ADHD/TL;DR crowd:

1. ATF ruled that a trust is not a person as defined by 18 U.S. Code § 921(a) and the Gun Control Act (GCA).

2. Since ATF holds that an unincorporated trust is not a “person” under the GCA, the prohibition on the transfer or possession of machineguns as defined by 18 U.S. Code § 921(a)(1) and 18 U.S. Code § 922(o) cannot apply to unincorporated trusts.

3. Numerous (See Footnote #1) trusts submitted Form 1 applications to build new machine guns.

4. ATF approved the applications and sent out stamps (See Footnote #2).

5. On or around 9/10/14, ATF began calling trusts (or, rather, Trustees) that received stamps and/or updating their online status from Approved to Disapproved. Those that were called were told they had to return the stamp. (See Footnote #3 for audio of one such call)

6. Attorneys are working on it now...

Footnotes:

1. Current estimates put the number of trusts that applied somewhere in the low hundreds. Unfortunately, there is no 100% accurate count that is publicly available.

2. Some stamps were received as early as the first week of August 2014. Others have not been received yet.

3. http://www.youtube.com/watch?v=4s9GKoxnGcM

In case anyone missed it, there's been some updates and a legal battle is about to begin.