curl -v -X POST -d "key=val" http://127.0.0.1:4567/postman/123456 * About to connect() to 127.0.0.1 port 4567 (#0) * Trying 127.0.0.1... connected * Connected to 127.0.0.1 (127.0.0.1) port 4567 (#0) > POST /postman/123456 HTTP/1.1 > User-Agent: curl/7.21.6 (x86_64-pc-linux-gnu) libcurl/7.21.6 OpenSSL/1.0.0e zlib/1.2.3.4 libidn/1.22 librtmp/2.3 > Host: 127.0.0.1:4567 > Accept: */* > Content-Length: 7 > Content-Type: application/x-www-form-urlencoded > < HTTP/1.1 404 Not Found < X-Frame-Options: sameorigin < X-Xss-Protection: 1; mode=block < Content-Type: text/html;charset=utf-8 < X-Cascade: pass < Content-Length: 451 < Server: WEBrick/1.3.1 (Ruby/1.9.2/2011-07-09) < Date: Fri, 02 Mar 2012 23:11:57 GMT < Connection: Keep-Alive <

Sinatra doesn’t know this ditty.

Try this: 
post '/postman/123456' do
  "Hello World"
end
* Connection #0 to host 127.0.0.1 left intact * Closing connection #0