Insulin resistance isn't a static thing that you have or you haven't, it's on a continuum. You have more of it or less of it, not none of it. There are some people with huge insulin resistance... there is one very rare type of diabetes where the sufferers need thousands of units of insulin a day to get their glucose levels approaching normal. These people are actually very thin as glucose doesn't get into the cells so they burn a lot of fat. Some very resistant type 2s when they go onto insulin may also need hundreds of units a day, yet may still be making some of their own . Some very insulin sensitive type ones may take less than 20. I know of one very active T1 who used to take just a few units but she was an ultra marathoner running hundred km races.
In all people it varies from day to day and over time. Adolescents can become more insulin resistant as do women in pregnancy . People who are ill may become more insulin resistant. On the other hand people who are sedentary and start an exercise regime can become more insulin sensitive (I find this is a day to day thing, no exercise for a couple of days and my insulin needs rise) . People who lose weight become more insulin sensitive.
There are lots of possible factors as to why people are more insulin resistant. Fat deposited around and in the organs (particularly liver and pancreas) results in insulin resistance. If you have high glucose levels this also increases insulin resistance (vicious circle..... It takes more insulin to lower the glucose by 1mmol at a level of 20mmol/l than it would do at 6mml/l. (glucosetoxicity). Similarly, excess fatty acids in the blood results in increased insulin resistance (lipotoxicity)
Insulin resistance in cells in the liver results in the over production of glucose there and contributes to high fasting glucose levels. Insulin resistance in the muscle and skeletal cells in the body means that the glucose can't get into the cells for use without extra insulin and if there isn't enough glucose levels will stay high.
Daibell: there is a lot of evidence to show insulin resistance in T1. Have a look at this study where the T1s had more hepatic (liver) insulin resistance than the T2s.
http://www.springerlink.com/content/p31406848115k767/