Hello Peter and All,
Peter wrote:
My understanding is that the damage to colonies is not caused by directly by varroa, but by the fact that they act as vectors for viruses.  Of these, the most significant are Deformed Wing Virus (DWV) and Acute Paralysis Virus (APV).  So a large infestation of varroa may not be a problem if DFW and APV are absent, whereas a small infestation can cause severe problems where these viruses are present.

Bailey has done a huge amount of research on the above and named many viruses. Dr. Norman Carrick of the U.K. is following in Baileys footsteps.  Quite a bit of information about viruses is in the archives of the Irish Beekeeping list. 

If I understand Norman correctly she has hives living with varroa as long as she can control the viruses  which is no easy task. I find both the work of Bailey and Dr. Carrick most interesting but still a few years away from being the answer for varroa control in my opinion but I might be wrong.
Peter wrote:
Should we, therefore, be monitoring the presence of the viruses (how do we do that?) rather than getting obsessed about the actual number of varroa present?
Diagnosis of viral infections in honey bees is beyound the expertise of most beekeepers BUT not all.
Counting the actual number of varroa is much easier for most beekeepers to deal with.
Excellent questions and comments Peter!

Sincerely,
Bob Harrison
Ps. Dr. Carrick sent me the graphs etc. that she was to give in South Africa to look at last spring.  I was very impressed with her work.  Maybe one of these days the answer for varroa might be in virus control but Bailey was interested in virus research as a cure for tracheal mites and only recently has the approach been applied to varroa.( last decade I believe) )