title: Recent-ish changes to the Python execution model?
body: <p>I just re-read the section on execution models in the 3rd edition of <em>Learning Python</em> (late 2007), and it felt fairly tentative. So, I looked at the same section in the 4th edition (late 2009) and was pretty disappointed that it was completely unchanged.</p>
  
  <p>What is the status for executing Python beyond CPython? It feels like Jython and IronPython are still niche projects; have other similar projects emerged? has Psyco solidified well enough to use without worry? Are many people using ShedSkin? Is there information on when PyPy is generally faster for execution?</p>