rspec users - Oct 2006 - how to spec elements of config file usage

Hi!

I''m pretty newbie in Ruby and certainly in rspec.  

I''m writing a wrapper to a 3rd party API in Ruby, and
will use a config file to persist the API user keys. 
In setting up my specs I want to say things like:

"should raise exception if config file not found"
"should raise exception if config file does not
contain key"

I figure that I''ll swap config files in the beginning
of each spec.

If I ''require rake'' in my spec script, I''ll be able
to
use the File methods from rake to swap out versions of
my config file - right?  Any reason not to do it that
way, i.e. some other Ruby library is better?

Other recommended methods to handle specs around
presence / contents of file system objects?

Thanks,

Pauli