Thanks, everyone! To the poster (who now appears to be banned) who seemed to imply that I was using this forum thinking I was getting advice from a licensed vet, rest assured that Miles was seen by a vet. However, it is nice to be able to come to a forum like this to get opinions and advice from others who may have been through similar things.
Miles is doing much better. It appears that it was stress colitis (I guess I can eat my words now!), and his stools are completely back to normal. Regardless of whether this is "normal" or not, I still maintain that they should have called me when it started (they even have a vet on-site and I have given written permission for Miles to be seen by the vet if warranted). And I cannot understand why they would have let him sit in his own diarrhea and still have him covered in it when I picked him up. I did e-mail the owner about the incident and hope to hear back soon!
Thanks again for all of your feedback. It is much appreciated!