This is a topic that is near and dear to my heart as I have a 15 year old mare who was diagnosed with Cushings almost 2 years ago. I also have 2 other mares who were diagnosed with Insulin Resistance (IR). What many people, and way too many vets do, is to confuse the two conditions. I am not an expert and will not do a technical dissertation here, but I do want to say that Cushings Disease is treated with Pergolide, where IR is treated by diet and excersize (when horse is able).
Some Cushings horses are ALSO IR, but all IR horses do NOT have Cushings Disease. Despite what some folks seem to be saying, it is not protocol to treat your overweight, cresty horses, who are most likely IR, with Pergolide.
I enthusiastically recommend the web site Keri has mentioned on this thread - the Yahoo group targetsmom has just joined. (You will love it there, Mary!) It ihas been my bible for the last two years. It is run by a very respected and published veterinarian who specializes in equine nutrition, and the site is light years ahead in research and information on Cushings and IR. You owe it to your horses to check out this site.
It should also be noted that minis are a breed that is prone to Insulin Resistance, and that untreated IR and Cushings can lead to laminitis - something to be avoided at all costs, as we know. In the case of one of my mares, she was never overweight, had no heavy crest or fat pads in those telltale places, but seemed always just a little 'ouchy'. After joining the Yahoo group for my Cushings mare, I had the other mare tested for glucose and insulin -- and her insulin levels were off the chart! Insulin resistant horses do not have to be 'fat'. I had been 'killing' her with sweet feed for years - like feeding a diabetic candy - and at that point, I changed my whole feeding program, and now everyone here - broodmares, stallions, babies, show horses - are on a low sugar/starch diet.
All my mares are doing very well now - the IR mares on their strict low starch/sugar diets with minerals balanced to their hay, and my Cushings girl well under control with a similar diet and daily Pergolide.
The Equine Cushings/IR site also has valuable info. on very reasonably priced sources for Pergolide capsules - the safest form, with the longest shelf life, in case your horse really does have Cushings Disease.
Well, I got a little carried away, after all.
This has become a passion of mine. Please do go take a look at the web site (Yahoo group) mentioned. It is THE place to learn all you ever wanted to know on this subject.
http://pets.groups.yahoo.com/group/EquineCushings/