Finding a script's home directory?

[Gabriel Cooper]

In one of my python programs has a data file I need to load. My solution 
was to say:

        if os.path.exists(os.path.join(os.getcwd(), "config.xml")):
            self.cfgfile = os.path.join(os.getcwd(), "config.xml")

Which works fine... as long as you're *in* the script's home directory 
when you run it (as in, run it as: ./startApp.py as opposed to 
./myApp/startApp.py).

If I run it from an alternate directory the program looks for the 
config.xml file in my current directory not the app's home directory. So 
how do I get the script's home directory?