Thank you for your advice.  I did a bit more reading...  The post you linked showed how to define a filter within a module. (I think :) Since there is only one field that I would like to change, I think I am looking for a way to change:
filter_form('format', $node->format);
to specify the wiki filter only. 

I tried this:
$output .= filter_form('format', $format=4);
which worked, since the Wiki Filter was the 4th option.  It doesn't seem quite dynamic.  Those without permissions won't see the php filter, if I add more filters somewhere, the wiki filter won't be the 4th one anymore.  Well, I suppose I could set it to always be, say, the 2nd one, but are there any better ways?

Anisa.

Bèr Kessels wrote:
Op zaterdag 5 maart 2005 15:04, schreef Anisa:
  
I would like to use a Wiki filter for a specific field in a new module I
am trying to make...  either make it a default for that one field, or
make the other filters unavailable.  Is there anyway to do this?  I
don't want to change it to the default for the whole site.
    

Hi,

There are lots of ways to do so.
A good place to start your Drupal coding is at drupal.org/contribute.
In your case you will be looking at filters and filter documentation. and 
because learning by example is (IMO) still the best way to do so: Have a look 
here: 
http://cvs.drupal.org/viewcvs/drupal/contributions/docs/developer/examples/filter_example.module?rev=1.6&view=log

Regards,
 Bèr