> SSL virtually requires buying certificates so
  > small/hobby sites won't use it

startssl.com offers free SSL certificates valid in almost every browser, good for one year. I've been using them for my own site with no problems. Certificate cost is no excuse for continuing to use unsecured HTTP.

  > it involves multiple round-trips and latency is never
  > going to go away

Assuming reasonable protocol design (somewhat problematic for HTTP/1.0, better in /1.1 and SPDY), additional round trips are only a factor for the initial connection setup. Later requests can re-use the existing SSL session.

  > part of what makes encryption work is that it's
  > computationally hard on today's hardware

Encryption is based on being computationally expensive for a third party. The computational load on the communicating parties is negligible, particularly with modern CPUs. From http://www.imperialviolet.org/2010/06/25/overclocking-ssl.ht... , Google experiences CPU overheads of less than one percent:

  > On our production frontend machines, SSL/TLS accounts
  > for less than 1% of the CPU load, less than 10KB of
  > memory per connection and less than 2% of network overhead.