Update
I wanted to take a minute to thank everyone who's responded with information, contacts, and support for us on this.
As it stands, outwardly, our daughter is the picture of health and happiness. INwardly, the doctors suspect she has a condition known as dyserythropoiesis. Her initial bone marrow test results seem to support this theory. I can't even pronounce it, but what it means is that her bone marrow is not producing enough healthy red blood cells that are making it into the bloodstream. Test results due in two weeks will tell us if it is genetic (and, therefore, requires a bone marrow transplant), or if it isn't. If it isn't, we're back to not knowing what has caused this. There is another test out, with results also due in a couple of weeks, looking for a wide range of viruses and infectious diseases. Hopefully either that result turns up something or this mysteriously heals itself and goes into the "Well, I'll be damned..." category.
The hospital here has a relationship with St. Jude's hospital in Memphis and is going to consult with them for a second opinion.
Thanks again for the help. You have no idea how much we have appreciated it. Pprune readers came through in spades for us and restored my faith in humanity.
Many, many thanks
Shawn