The ZRTP part is fine if you can bring your own client. And I guess $12 a month isn't much to pay for SC to run FreeSWITCH as a NAT traversal/endpoint discovery platform.
There's still no real details on how we're supposed to trust Silent Circle from being actively attacked and backdooring clients. If memory serves me correctly Mr. Zimmerman said they're relying on the government not being able to compel them to cooperate. AKA, Lavabit-style. (Sure, ZRTP is solid, but a compromised client isn't detectable to end-users, plus SC talks a lot about calling the PSTN.)
For a comparison to a company that takes security seriously, look at tarsnap. Then go read the PR from Mike Janke the other day.
On another note, what does "100% dedicated network – no sharing or leasing" even mean? Did SC run fibre from Toronto to Switzerland? How is SC's network any different from any other VoIP company that builds a datacenter?