We are seeing something similar - no "Send" option for PIN messages. (yes, the To: field is populated
Our issue seems to be only on new AT&T-branded 8700g with 188.8.131.52 version OS, and only after the Enterprise Activation with our BES (running v. 4.1.x.x). Before EA, the Send option appears on the compose PIN menu.
We've ruled out the IT Policy, because a newly-activated 8700g with version 184.108.40.2067 OS is fine.