Finally figured out my caching problem!

From the Sinatra documentation (http://www.sinatrarb.com/intro), the
find_template helper function will, in production mode only, cache not
only templates but also the template's content.

Since I was using the markdown filter provided by Sinatra and
rendering pages through Sinatra's templating, it was causing my
production instance (which additionally uses Apache and Passenger, rather
than WEBrick like my development box does) to never update pages.

The fix I came up with (using Redcarpet directly) also happens to work
around a Tilt bug I was also being affected by (not able to pass options
other than :filter_html to the renderer).

Lastly, I added partial support for what I'll probably call "namespaces"
(essentialy subdirectories).  I'll finish this up soon.