FPGAs probably make more sense for routers, because you want the ability to change the firmware more often, and a router has a bunch of other parts as well, and realistically, cypher-cracking is not an economically viable activity for most people, so the cost-benefit tradeoffs are a bit twisted.
The router world seems to use a good mixture. At a startup we were purchasing nice off-the-shelf MPLS ASICs, which did MPLS route setup and forwarding (and some enforcement) while the 'software'/control plane (eg, OSPF, RSVP-TE, etc...) was largely in FPGAs of our own brew.
At that time (ca, 2000/2001) some vendors were starting to push net processors, which were somewhere in between, and at the time just weren't quite fast enough for ASIC-busting applications and not quite flexible enough for FPGA-ish applications. Now, however, I'd bet net processors are very effective for metro-edge applications.
What I suspect is that there's already some crypto net processors out there, though they may be classified, or the commercial equivalent (ie, I assume there are 'classified' catalogs from companies like General Dynamics that normal clients never see). They can periodically upgrade the code when they discover that some new form of stego (for instance) has become in-vogue at Al Qaeda.
These won't be Variola Suitcase-type applications, though, but perhaps for special situations where they know the few locations in Cobble Hill Brooklyn they want to monitor and decrypt.
-TD
