In setting up Exchange 2007 OWA through ISA Server 2006, I realized that it would be a lot easier to tell users to access it via http://owa.domain.com as opposed to http://owa.domain.com/owa (which is repetitive if nothing else).
Problem is, there is no obvious way to do this in ISA or Exchange. Fortunately I am not the first one to have this problem, and it has already been solved. The short version of the fix is to set OWA up 100% according to whatever book, article, white paper you are already following. THEN, add another web access rule point to the same site, except this time set it to Deny. You don't have to get too fancy on the various settings, authentication, etc. and just use the same listener that you created for OWA.
Once the rule is created, make sure it is the one just above the main OWA access rule (I named the new one "Redirect OWA" so I would know what it was), and then go back in and on the deny page, set it to Redirect to your specific OWA subdirectory (http://owa.domain.com/owa). It works great!
Here are the two resources that I found to be most helpful:
Blog du Tristank (look in the comments)
**UPDATE** I have since found a blog that tells the correct way to do this (but the quick and dirty listed above still works fine). Here is a link to my blog post about the correct way.