This isn't true anymore. Octopress uses an older version of Jekyll. If you're on 3.0 (which we're using) has incremental generation. Here's the log of a recent incremental build:

  [Step 1/3] Jekyll build (11s)
  [20:52:40][Jekyll build] Jekyll build --config '_blog.config.yml' --no-watch
  [20:52:43][Jekyll build] Configuration file: C:/BuildAgent/work/4e27a39603eb2bf1/blog/_config.yml
  [20:52:43][Jekyll build]             Source: C:/BuildAgent/work/4e27a39603eb2bf1/blog
  [20:52:43][Jekyll build]        Destination: C:/BuildAgent/work/4e27a39603eb2bf1/blog/_site
  [20:52:43][Jekyll build]  Incremental build: disabled
  [20:52:43][Jekyll build]       Generating... 
  [20:52:52][Jekyll build]                     done in 9.082 seconds.