Depression caused by a virus?

In a recent thread, somebody mentioned a theory that depression is caused by the Borna virus, a virus normally associated with horses and cats (and some other animals). I found this interesting, since ulcers (earlier thought to have psychological causes) have been found to be caused by a bacteria, Helicobacter pylori.

What is the current state of research on this theory? Has it been disproven, or is research still going on? How likely is it that a virus could have a role in depression, and how important a factor could it be? What would be the implications if it turned out to be true?

Googling for “borna virus” will give you links to a number of research papers on the subject, but nothing really conclusive.

I know that most of the work on this has been done in the UK.
Does that help?