As an update to the post that started this thread, I have learned alot at the Yahoo Group for Cushings and Insulin Resistance (also called Equine Metabolic Syndrome). As some very basic info for readers, you can have IR without being Cushings and be Cushings without having IR - though many Cushings horses
also develop IR.
My mare has the lumpy fat pockets, cresty neck but not the long/coarse/curly coat, not the excessive sweating or lack of sweating. After discussion with several vets and others who have much experience with Cushings and IR - we are treating her for IR as part of the diagnosis. Testing for IR is imperfect at best currently. If the treatment works to resolve her issues, we will consider that proof positive that she is IR and continue the treatment that works and tweak it as we go. If it doesn't resolve her issues, then we will re-evaluate and go from there.
For me, since I'm currently boarding my horses out, I was unable to follow many of the suggestions of the specialized Yahoo! group on the topic. But after great encouragement from Amanda and my vets, we have mapped a plan. She receives a very strict diet in both amount of food and what food she gets - 1.5% of her body weight in forage per day consisting of low quality hay. For me this means 3 flakes prairie hay and 1 flake of brome a day. (We weighed flakes to get this formula.)
Then, to address the suggested supplements and various vitamins, etc - she receives 1# twice a day of Nutrena LiteBalance, a grain feed formulated for IR type horses. Now, this met with alot of resistance on yahoo! but is one of those compromise deals for what my barn owner was willing to do for work to feed my horse each day!

The LiteBalance appears rather bland and does not have the sweet smelling effect - the mice even seem to leave it alone. I liked it because it contains magnesium which I kept reading was a major factor in helping IR horses. This runs abour $15 per 40# bag in my area of Kansas.
She also gets 1# of ontario dehy timothy cubes twice a day. These are fomulated for IR horses as well and have received lots of endorsements over other types of timothy cubes, pellets, etc. This runs me about $10 per 50# bag in my area of Kansas.
Lastly, she gets roughly 2 tablespoons of chaste tree berry powder (aka vitex) per day. This cost me $12 for the bag and shipping and I have no idea how long it will last, but it looks like over a month at least.
We started the new feeding regimen on November 2nd at @ 1035# (per weight tape) and as of November 14 she was down to 938# (per weight tape). Now granted, the weight tape is nowhere near exact science, but if she's down again this weekend, I will consider it to be working. An appropriate weight for my mare would be around 800#. When she gets to that range - and hopefully loses the fat pads - if she is still losing weight, we will adjust the diet to include a little bit more of each component until we get her holding steady. She has a great winter coat (not excessive, it's comparable to the other horses at the barn) and while she's not so eager to get her feed anymore, she does eventually eat it all before the next feeding time comes around. (She's also not wild about her sugar free treats now.)
We go for a re-check at the vet in late December/early January to get the all clear for breeding. I have not found any research nor proof to show that IR is an inheritable trait or such a scourge on the horse to NOT breed her. I plan to try for an ET (g-d willing and pocketbook too!) but other than needing to manage her more closely and to remove her from the chaste tree berry (not safe for use in pregnant mares) - she is of good conformation, impeccable bloodlines and she is one of the prettiest and sweetest mares I've owned. I would not hesitate to breed her even if you guaranteed me the foal would be an IR horse too- I don't say this lightly, but it is proving to be a manageable issue, not a death sentence.
Stephanie