There are quite a few threads about diabetic cats here, and one discusses using a low-carb, high-protein diet, which I would whole-heartedly support. I'm a diabetic human, and it works wonders (of course, I'm a tad bit more disciplined with the eating than a cat). Dr. Bernstein's online forum
is full of people who follow LC-management for diabetes - and a few with diabetic cats for patients too. If you're cat is skinny, an LC diet might cause problems if it is TOO low-carb - I don't know about cats, but in humans, insulin is neccessary to gain fat, as insulin supresses the enzyme which breaks down fat for energy instead of storage (lipase) - at least in humans - cats are probably slightly different.
A cat's normal range as far as I know is the same as a human's which is 70-90 mg/dl (although a cat's can apparently be much wider - all the way to 400 at times I think). I had my cat's BG checked when I though it might be diabetic (it had been acting a bit more lethargic than usual..), and it was normal at 72.
The human version of the LC-diet is low-carb, MODERATE-protein, high-FAT (about 10-40-50 %). The fat will break down into ketones, which can be used as metabolic fuel by (most of) the brain, the heart, and probably most other vital organs. In humans at least, this appears to lower the threshold of "severe hypoglycemia" (seizures, loss of conscieousness) a LOT - I can go down to 30 mg/dl (normal is 83), and still be functional and lucid. The danger here is ketoacidosis - which I assume would be greater for cats as you can't monitor them as much as people. As long as insulin is adequate and blood glucose is kept normal, ketoacidosis will not occur - but that might be hard with a cat. As long as insulin is adequate to prevent BG from rising too far, this should not occur - I'd think the easiest way to do this would be a long-acting basal insulin such as NPH or Lantus I'm not sure if NPH will work safely in a cat though. NPH also has a pronounced peak unless the dose is very small - I assume, I don't use NPH.
So I don't know if a high-fat diet would really be appropriate for a cat.. I'm told taht most diabetic cat's are similar to human Type 2 (so there is little if any beta-cell deficit), and many will somehow go into remission after about a year (humans apparently got short-shrift when it came to our regeneratory systems). A low-carb diet of any kind will help this, as it puts less stress on what beta-cells are there.