Real terrorists have already been using that for a long time. (For instance, remember those media blurbs about lots of porn being found in the raid on bin Laden?) As such I'd bet they have infrastructure in place to detect such communications.

But in the same half-joking vein, some other problems:

1. I recall reading that such stegonagraphically watermarked videos were pretty easy to detect using simple statistical analysis of the noise in the frames. With dedicated hardware for multimedia decoding cheaply and ubiquitously available, frames can be efficiently extracted and the additional number of bytes does little to slow them down. If the hidden data is encrypted, well, you could just do that without the porn.

2. Unless you make it as easy as tapping a key to start communicating, it still does not remove a main barrier to widespread adoption of encryption, which is inconvenience.

3. The problem with porn is, people will just end up paying more attention to the medium rather than the message.