Check out the Administration Menu module (http://drupal.org/project/admin_menu).  It allows you to switch users without logging out.

Steve

Christopher M. Jones wrote: 
Is there a module that enables me to switch user and / or role without 
logging out and back in? The one aspect of D development that bugs me 
most is logging in and out to test permissions settings and whatnot.

I looked on drupal.org and didn't see anything. Maybe I missed it.